diff --git a/Classes/Service/AnalyticsStatusService.php b/Classes/Service/AnalyticsStatusService.php
index b3379dd..4ee03ab 100644
--- a/Classes/Service/AnalyticsStatusService.php
+++ b/Classes/Service/AnalyticsStatusService.php
@@ -9,6 +9,7 @@
 use Psr\Log\LoggerInterface;
 use T3G\Analytics\Exception\AnalyticsApiException;
 use TYPO3\CMS\Core\Cache\Frontend\FrontendInterface;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Site\Entity\Site;
 use TYPO3\CMS\Core\Site\SiteSettingsFactory;
 use TYPO3\CMS\Core\Site\SiteSettingsService;
@@ -25,6 +26,7 @@ public function __construct(
         private LoggerInterface $logger,
         private SiteSettingsService $siteSettingsService,
         private SiteSettingsFactory $siteSettingsFactory,
+        private SiteSettingsWriteVerifierInterface $writeGuard,
     ) {
     }
 
@@ -110,7 +112,26 @@ public function syncSiteSettingsFromStatus(Site $site, array $data): void
         }
 
         $existing = $this->siteSettingsFactory->loadLocalSettings($site->getIdentifier()) ?? [];
-        $this->siteSettingsService->writeSettings($site, array_merge($existing, $update));
+        try {
+            $this->siteSettingsService->writeSettings($site, array_merge($existing, $update));
+        } catch (SiteConfigurationWriteException $e) {
+            $this->logger->warning(
+                'syncSiteSettingsFromStatus: writeSettings threw. Check file system permissions.',
+                ['siteIdentifier' => $site->getIdentifier(), 'exception' => $e->getMessage()]
+            );
+            return;
+        }
+
+        try {
+            $this->writeGuard->assertSettingsPersisted($site, $update);
+        } catch (AnalyticsApiException) {
+            $this->logger->warning(
+                'syncSiteSettingsFromStatus: settings could not be persisted. Check file system permissions.',
+                ['siteIdentifier' => $site->getIdentifier()]
+            );
+            return;
+        }
+
         $this->logger->info(
             'Site settings updated from status response.',
             ['siteIdentifier' => $site->getIdentifier(), 'update' => $update]
diff --git a/Classes/Service/ApiKeyService.php b/Classes/Service/ApiKeyService.php
index af4f036..a0e8f1a 100644
--- a/Classes/Service/ApiKeyService.php
+++ b/Classes/Service/ApiKeyService.php
@@ -8,6 +8,7 @@
 use Psr\Container\NotFoundExceptionInterface;
 use Psr\Log\LoggerInterface;
 use T3G\Analytics\Exception\AnalyticsApiException;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Site\Entity\Site;
 use TYPO3\CMS\Core\Site\SiteSettingsFactory;
 use TYPO3\CMS\Core\Site\SiteSettingsService;
@@ -19,6 +20,7 @@ public function __construct(
         private CipherServiceInterface $cipherService,
         private SiteSettingsService $siteSettingsService,
         private SiteSettingsFactory $siteSettingsFactory,
+        private SiteSettingsWriteVerifierInterface $writeGuard,
         private LoggerInterface $logger,
     ) {
     }
@@ -102,10 +104,28 @@ public function provisionIfNeeded(Site $site, array $currentStatus): void
         }
 
         $existing = $this->siteSettingsFactory->loadLocalSettings($siteIdentifier) ?? [];
-        $this->siteSettingsService->writeSettings($site, array_merge($existing, [
-            'apiKeyId' => $result['apiKeyId'],
-            'apiKey' => $encryptedApiKey,
-        ]));
+        try {
+            $this->siteSettingsService->writeSettings($site, array_merge($existing, [
+                'apiKeyId' => $result['apiKeyId'],
+                'apiKey' => $encryptedApiKey,
+            ]));
+        } catch (SiteConfigurationWriteException $e) {
+            $this->logger->error('ApiKeyService: Failed to write API key settings to site configuration.', [
+                'siteIdentifier' => $siteIdentifier,
+                'exception' => $e->getMessage(),
+            ]);
+            return;
+        }
+
+        try {
+            $this->writeGuard->assertSettingsPersisted($site, ['apiKeyId' => $result['apiKeyId']]);
+        } catch (AnalyticsApiException $e) {
+            $this->logger->error('ApiKeyService: API key could not be persisted.', [
+                'siteIdentifier' => $siteIdentifier,
+                'apiKeyId' => $result['apiKeyId'],
+            ]);
+            return;
+        }
 
         $this->logger->info('ApiKeyService: API key provisioned.', [
             'siteIdentifier' => $siteIdentifier,
diff --git a/Classes/Service/InstanceRegistrationService.php b/Classes/Service/InstanceRegistrationService.php
index 394eaf8..e3cab37 100644
--- a/Classes/Service/InstanceRegistrationService.php
+++ b/Classes/Service/InstanceRegistrationService.php
@@ -6,6 +6,7 @@
 
 use Psr\Log\LoggerInterface;
 use T3G\Analytics\Exception\AnalyticsApiException;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Site\Entity\Site;
 use TYPO3\CMS\Core\Site\SiteSettingsFactory;
 use TYPO3\CMS\Core\Site\SiteSettingsService;
@@ -17,6 +18,7 @@ public function __construct(
         private CipherServiceInterface $cipherService,
         private SiteSettingsService $siteSettingsService,
         private SiteSettingsFactory $siteSettingsFactory,
+        private SiteSettingsWriteVerifierInterface $writeGuard,
         private LoggerInterface $logger,
     ) {
     }
@@ -31,6 +33,8 @@ public function register(Site $site, string $email): void
     {
         $siteIdentifier = $site->getIdentifier();
 
+        $this->writeGuard->assertDirectoryWritable($site);
+
         try {
             $data = $this->apiClient->registerInstance($site, $email);
         } catch (AnalyticsApiException $e) {
@@ -51,11 +55,23 @@ public function register(Site $site, string $email): void
         $encryptedSecret = $instanceSecret !== '' ? $this->cipherService->encrypt($instanceSecret) : '';
 
         $existing = $this->siteSettingsFactory->loadLocalSettings($siteIdentifier) ?? [];
-        $this->siteSettingsService->writeSettings($site, array_merge($existing, [
-            'websiteId' => $websiteId,
-            'instanceId' => $instanceId,
-            'instanceSecret' => $encryptedSecret,
-        ]));
+        try {
+            $this->siteSettingsService->writeSettings($site, array_merge($existing, [
+                'websiteId' => $websiteId,
+                'instanceId' => $instanceId,
+                'instanceSecret' => $encryptedSecret,
+            ]));
+        } catch (SiteConfigurationWriteException $e) {
+            $this->logger->error('Registration: writeSettings threw.', ['siteIdentifier' => $siteIdentifier, 'exception' => $e->getMessage()]);
+            throw new AnalyticsApiException('Settings could not be written to config/sites/' . $siteIdentifier . '/settings.yaml. Check file system permissions for this directory.', 0);
+        }
+
+        try {
+            $this->writeGuard->assertSettingsPersisted($site, ['websiteId' => $websiteId]);
+        } catch (AnalyticsApiException $e) {
+            $this->logger->error('Registration: settings could not be persisted.', ['siteIdentifier' => $siteIdentifier]);
+            throw $e;
+        }
 
         $this->logger->info('Site successfully registered.', ['siteIdentifier' => $siteIdentifier, 'websiteId' => $websiteId]);
     }
diff --git a/Classes/Service/SiteSettingsWriteVerifier.php b/Classes/Service/SiteSettingsWriteVerifier.php
new file mode 100644
index 0000000..22486cb
--- /dev/null
+++ b/Classes/Service/SiteSettingsWriteVerifier.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace T3G\Analytics\Service;
+
+use T3G\Analytics\Exception\AnalyticsApiException;
+use TYPO3\CMS\Core\Core\Environment;
+use TYPO3\CMS\Core\Site\Entity\Site;
+use TYPO3\CMS\Core\Site\SiteSettingsFactory;
+
+final readonly class SiteSettingsWriteVerifier implements SiteSettingsWriteVerifierInterface
+{
+    public function __construct(
+        private SiteSettingsFactory $siteSettingsFactory,
+    ) {
+    }
+
+    public function assertDirectoryWritable(Site $site): void
+    {
+        $identifier = $site->getIdentifier();
+        $configDir = Environment::getConfigPath() . '/sites/' . $identifier;
+
+        if (!is_dir($configDir) || !is_writable($configDir)) {
+            throw new AnalyticsApiException(
+                'Cannot write to site configuration directory for "' . $identifier . '". Check file system permissions.',
+                0
+            );
+        }
+    }
+
+    public function assertSettingsPersisted(Site $site, array $expected): void
+    {
+        $identifier = $site->getIdentifier();
+        $persisted = $this->siteSettingsFactory->loadLocalSettings($identifier) ?? [];
+
+        foreach ($expected as $key => $value) {
+            if (($persisted[$key] ?? '') !== $value) {
+                throw new AnalyticsApiException(
+                    'Settings for site "' . $identifier . '" could not be persisted. Check file system permissions.',
+                    0
+                );
+            }
+        }
+    }
+
+}
diff --git a/Classes/Service/SiteSettingsWriteVerifierInterface.php b/Classes/Service/SiteSettingsWriteVerifierInterface.php
new file mode 100644
index 0000000..4eea638
--- /dev/null
+++ b/Classes/Service/SiteSettingsWriteVerifierInterface.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace T3G\Analytics\Service;
+
+use T3G\Analytics\Exception\AnalyticsApiException;
+use TYPO3\CMS\Core\Site\Entity\Site;
+
+interface SiteSettingsWriteVerifierInterface
+{
+    /**
+     * Asserts that the site configuration directory is writable.
+     *
+     * @throws AnalyticsApiException
+     */
+    public function assertDirectoryWritable(Site $site): void;
+
+    /**
+     * Asserts that the given settings were actually persisted to disk.
+     * Call this after writeSettings() to detect silent write failures.
+     *
+     * @param array<string, mixed> $expected key-value pairs that must appear in the persisted settings
+     * @throws AnalyticsApiException
+     */
+    public function assertSettingsPersisted(Site $site, array $expected): void;
+}
diff --git a/Configuration/Services.yaml b/Configuration/Services.yaml
index fa2d07a..b9d16de 100644
--- a/Configuration/Services.yaml
+++ b/Configuration/Services.yaml
@@ -12,6 +12,9 @@ services:
       - '../Classes/Dashboard/Widget/TrafficGraphWidgetV14.php'
       - '../Classes/Dashboard/Widget/TrafficSourcesWidgetV14.php'
 
+  T3G\Analytics\Service\SiteSettingsWriteVerifierInterface:
+    alias: T3G\Analytics\Service\SiteSettingsWriteVerifier
+
   T3G\Analytics\Service\TopPagesServiceInterface:
     alias: T3G\Analytics\Service\TopPagesService
 
diff --git a/Tests/Functional/Controller/BackendModuleControllerTest.php b/Tests/Functional/Controller/BackendModuleControllerTest.php
index 1518419..39ba3db 100644
--- a/Tests/Functional/Controller/BackendModuleControllerTest.php
+++ b/Tests/Functional/Controller/BackendModuleControllerTest.php
@@ -22,6 +22,7 @@
 use T3G\Analytics\Service\ApiKeyService;
 use T3G\Analytics\Service\InstanceRegistrationService;
 use T3G\Analytics\Service\SiteDataProvider;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use T3G\Analytics\Service\BackendPageAccessCheckerInterface;
 use T3G\Analytics\Tests\Functional\Bootstrap\FunctionalTestCase;
 use TYPO3\CMS\Backend\Routing\Route;
@@ -415,6 +416,7 @@ private function buildController(
             new NullLogger(),
             $this->createMock(SiteSettingsService::class),
             $this->createMock(SiteSettingsFactory::class),
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
         );
 
         $siteDataProvider = new SiteDataProvider(
@@ -436,6 +438,7 @@ private function buildController(
             $cipherService,
             $this->createMock(SiteSettingsService::class),
             $this->createMock(SiteSettingsFactory::class),
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
             new NullLogger(),
         );
 
@@ -444,6 +447,7 @@ private function buildController(
             $cipherService,
             $this->createMock(SiteSettingsService::class),
             $this->createMock(SiteSettingsFactory::class),
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
             new NullLogger(),
         );
 
diff --git a/Tests/Functional/Service/AnalyticsStatusServiceTest.php b/Tests/Functional/Service/AnalyticsStatusServiceTest.php
index aec684d..44ddf0f 100644
--- a/Tests/Functional/Service/AnalyticsStatusServiceTest.php
+++ b/Tests/Functional/Service/AnalyticsStatusServiceTest.php
@@ -15,6 +15,7 @@
 use T3G\Analytics\Service\ApiExceptionExtractor;
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use T3G\Analytics\Tests\Functional\Bootstrap\FunctionalTestCase;
 use TYPO3\CMS\Core\Cache\CacheManager;
 use TYPO3\CMS\Core\Cache\Frontend\FrontendInterface;
@@ -73,6 +74,7 @@ protected function setUp(): void
             new \Psr\Log\NullLogger(),
             $this->createMock(SiteSettingsService::class),
             $this->createMock(SiteSettingsFactory::class),
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
         );
     }
 
diff --git a/Tests/Functional/Service/InstanceRegistrationServiceTest.php b/Tests/Functional/Service/InstanceRegistrationServiceTest.php
index edc4a77..33f3deb 100644
--- a/Tests/Functional/Service/InstanceRegistrationServiceTest.php
+++ b/Tests/Functional/Service/InstanceRegistrationServiceTest.php
@@ -16,6 +16,7 @@
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
 use T3G\Analytics\Service\InstanceRegistrationService;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use T3G\Analytics\Tests\Functional\Bootstrap\FunctionalTestCase;
 use TYPO3\CMS\Core\Http\Client\GuzzleClientFactory;
 use TYPO3\CMS\Core\Http\RequestFactory;
@@ -75,6 +76,7 @@ protected function setUp(): void
             $this->cipherService,
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
             new \Psr\Log\NullLogger(),
         );
     }
diff --git a/Tests/Functional/Service/SiteSettingsWriteVerifierTest.php b/Tests/Functional/Service/SiteSettingsWriteVerifierTest.php
new file mode 100644
index 0000000..0a71a96
--- /dev/null
+++ b/Tests/Functional/Service/SiteSettingsWriteVerifierTest.php
@@ -0,0 +1,96 @@
+<?php
+
+declare(strict_types=1);
+
+namespace T3G\Analytics\Tests\Functional\Service;
+
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\MockObject\MockObject;
+use T3G\Analytics\Exception\AnalyticsApiException;
+use T3G\Analytics\Service\SiteSettingsWriteVerifier;
+use T3G\Analytics\Tests\Functional\Bootstrap\FunctionalTestCase;
+use TYPO3\CMS\Core\Http\Uri;
+use TYPO3\CMS\Core\Settings\Settings;
+use TYPO3\CMS\Core\Site\Entity\Site;
+use TYPO3\CMS\Core\Site\Entity\SiteSettings;
+use TYPO3\CMS\Core\Site\SiteSettingsFactory;
+
+final class SiteSettingsWriteVerifierTest extends FunctionalTestCase
+{
+    private SiteSettingsFactory&MockObject $siteSettingsFactory;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->siteSettingsFactory = $this->createMock(SiteSettingsFactory::class);
+    }
+
+    #[Test]
+    public function assertSettingsPersistedDoesNotThrowWhenAllValuesMatch(): void
+    {
+        $this->expectNotToPerformAssertions();
+
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([
+            'websiteId' => 'w-123',
+            'instanceId' => 'i-456',
+        ]);
+        $verifier = new SiteSettingsWriteVerifier($this->siteSettingsFactory);
+
+        $verifier->assertSettingsPersisted($this->buildSite(), ['websiteId' => 'w-123', 'instanceId' => 'i-456']);
+    }
+
+    #[Test]
+    public function assertSettingsPersistedThrowsWhenKeyIsAbsent(): void
+    {
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $verifier = new SiteSettingsWriteVerifier($this->siteSettingsFactory);
+
+        $this->expectException(AnalyticsApiException::class);
+        $this->expectExceptionMessageMatches('/"main"/');
+
+        $verifier->assertSettingsPersisted($this->buildSite(), ['websiteId' => 'w-123']);
+    }
+
+    #[Test]
+    public function assertSettingsPersistedThrowsWhenValueDoesNotMatch(): void
+    {
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn(['websiteId' => 'other-id']);
+        $verifier = new SiteSettingsWriteVerifier($this->siteSettingsFactory);
+
+        $this->expectException(AnalyticsApiException::class);
+
+        $verifier->assertSettingsPersisted($this->buildSite(), ['websiteId' => 'w-123']);
+    }
+
+    #[Test]
+    public function assertSettingsPersistedThrowsOnFirstFailingKey(): void
+    {
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn(['websiteId' => 'w-123']);
+        $verifier = new SiteSettingsWriteVerifier($this->siteSettingsFactory);
+
+        $this->expectException(AnalyticsApiException::class);
+
+        $verifier->assertSettingsPersisted($this->buildSite(), ['websiteId' => 'w-123', 'instanceId' => 'i-456']);
+    }
+
+    #[Test]
+    public function assertSettingsPersistedReadsSettingsOnlyOnce(): void
+    {
+        $this->siteSettingsFactory
+            ->expects(self::once())
+            ->method('loadLocalSettings')
+            ->willReturn(['websiteId' => 'w-123', 'instanceId' => 'i-456']);
+
+        $verifier = new SiteSettingsWriteVerifier($this->siteSettingsFactory);
+        $verifier->assertSettingsPersisted($this->buildSite(), ['websiteId' => 'w-123', 'instanceId' => 'i-456']);
+    }
+
+    private function buildSite(): Site
+    {
+        $site = $this->createMock(Site::class);
+        $site->method('getIdentifier')->willReturn('main');
+        $site->method('getBase')->willReturn(new Uri('https://example.com'));
+        $site->method('getSettings')->willReturn(new SiteSettings(new Settings([]), [], []));
+        return $site;
+    }
+}
diff --git a/Tests/Unit/Controller/BackendModuleControllerTest.php b/Tests/Unit/Controller/BackendModuleControllerTest.php
index 2fa4e1e..ac49de2 100644
--- a/Tests/Unit/Controller/BackendModuleControllerTest.php
+++ b/Tests/Unit/Controller/BackendModuleControllerTest.php
@@ -24,6 +24,7 @@
 use T3G\Analytics\Service\HmacSigner;
 use T3G\Analytics\Service\InstanceRegistrationService;
 use T3G\Analytics\Service\SiteDataProvider;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use T3G\Analytics\Service\BackendPageAccessCheckerInterface;
 use TYPO3\CMS\Core\Database\ConnectionPool;
 use TYPO3\CMS\Core\Database\Query\QueryBuilder;
@@ -136,6 +137,7 @@ protected function setUp(): void
             new NullLogger(),
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
         );
 
         $registrationService = new InstanceRegistrationService(
@@ -143,6 +145,7 @@ protected function setUp(): void
             $cipherService,
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
             new NullLogger(),
         );
 
@@ -151,6 +154,7 @@ protected function setUp(): void
             $cipherService,
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
             new NullLogger(),
         );
 
diff --git a/Tests/Unit/Service/AnalyticsStatusServiceTest.php b/Tests/Unit/Service/AnalyticsStatusServiceTest.php
index 86e2ca8..a3abb06 100644
--- a/Tests/Unit/Service/AnalyticsStatusServiceTest.php
+++ b/Tests/Unit/Service/AnalyticsStatusServiceTest.php
@@ -12,13 +12,16 @@
 use PHPUnit\Framework\MockObject\MockObject;
 use Psr\Log\NullLogger;
 use T3G\Analytics\Configuration\ApiConfiguration;
+use T3G\Analytics\Exception\AnalyticsApiException;
 use T3G\Analytics\Service\AnalyticsApiClient;
 use T3G\Analytics\Service\AnalyticsStatusService;
 use T3G\Analytics\Service\ApiExceptionExtractor;
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use TYPO3\CMS\Core\Cache\Backend\TransientMemoryBackend;
 use TYPO3\CMS\Core\Cache\Frontend\VariableFrontend;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Information\Typo3Version;
 use TYPO3\CMS\Core\Http\Client\GuzzleClientFactory;
 use TYPO3\CMS\Core\Http\RequestFactory;
@@ -38,6 +41,7 @@ final class AnalyticsStatusServiceTest extends UnitTestCase
     private array $httpHistory = [];
     private SiteSettingsService&MockObject $siteSettingsService;
     private SiteSettingsFactory&MockObject $siteSettingsFactory;
+    private SiteSettingsWriteVerifierInterface&MockObject $writeGuard;
 
     private VariableFrontend $cache;
     private string $encryptedTestSecret;
@@ -58,6 +62,7 @@ protected function setUp(): void
 
         $this->siteSettingsService = $this->createMock(SiteSettingsService::class);
         $this->siteSettingsFactory = $this->createMock(SiteSettingsFactory::class);
+        $this->writeGuard = $this->createMock(SiteSettingsWriteVerifierInterface::class);
         // TransientMemoryBackend dropped the $context parameter in TYPO3 v14.
         $backend = (new Typo3Version())->getMajorVersion() >= 14
             ? new TransientMemoryBackend() // @phpstan-ignore argument.count
@@ -84,6 +89,7 @@ protected function setUp(): void
             new NullLogger(),
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->writeGuard,
         );
     }
 
@@ -231,6 +237,39 @@ public function skipsApiCallOnCacheHit(): void
         self::assertEmpty($this->httpHistory);
     }
 
+    #[Test]
+    public function syncSiteSettingsFromStatusLogsWarningWhenWriteSettingsThrows(): void
+    {
+        $site = $this->buildSite('main', 'w-123', 'i-456');
+        $this->mockHandler->append(new Response(200, [], '{"status":"active","maxPrivacyModeTrackingCode":"tc-abc"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->siteSettingsService
+            ->method('writeSettings')
+            ->willThrowException(new SiteConfigurationWriteException('disk full', 1590487411));
+
+        $status = $this->subject->getStatus($site, forceRefresh: true);
+        $this->subject->syncSiteSettingsFromStatus($site, $status ?? []);
+
+        $this->expectNotToPerformAssertions();
+    }
+
+    #[Test]
+    public function syncSiteSettingsFromStatusLogsWarningWhenSettingsCouldNotBePersisted(): void
+    {
+        $site = $this->buildSite('main', 'w-123', 'i-456');
+        $this->mockHandler->append(new Response(200, [], '{"status":"active","maxPrivacyModeTrackingCode":"tc-abc"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->writeGuard
+            ->method('assertSettingsPersisted')
+            ->willThrowException(new AnalyticsApiException('Credentials could not be written.', 0));
+
+        $status = $this->subject->getStatus($site, forceRefresh: true);
+        $this->subject->syncSiteSettingsFromStatus($site, $status ?? []);
+
+        // No exception propagated — the service warns and returns.
+        $this->expectNotToPerformAssertions();
+    }
+
     /** Helpers */
 
     private function buildSite(string $identifier, string $websiteId, string $instanceId, array $extraSettings = []): Site
diff --git a/Tests/Unit/Service/ApiKeyServiceTest.php b/Tests/Unit/Service/ApiKeyServiceTest.php
index dcbb762..1c4d15a 100644
--- a/Tests/Unit/Service/ApiKeyServiceTest.php
+++ b/Tests/Unit/Service/ApiKeyServiceTest.php
@@ -12,11 +12,14 @@
 use PHPUnit\Framework\MockObject\MockObject;
 use Psr\Log\NullLogger;
 use T3G\Analytics\Configuration\ApiConfiguration;
+use T3G\Analytics\Exception\AnalyticsApiException;
 use T3G\Analytics\Service\AnalyticsApiClient;
 use T3G\Analytics\Service\ApiExceptionExtractor;
 use T3G\Analytics\Service\ApiKeyService;
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Http\Client\GuzzleClientFactory;
 use TYPO3\CMS\Core\Http\RequestFactory;
 use TYPO3\CMS\Core\Settings\Settings;
@@ -34,6 +37,7 @@ final class ApiKeyServiceTest extends UnitTestCase
     private array $httpHistory = [];
     private SiteSettingsService&MockObject $siteSettingsService;
     private SiteSettingsFactory&MockObject $siteSettingsFactory;
+    private SiteSettingsWriteVerifierInterface&MockObject $writeGuard;
     private CipherService $cipherService;
     private ApiKeyService $subject;
 
@@ -56,6 +60,7 @@ protected function setUp(): void
 
         $this->siteSettingsService = $this->createMock(SiteSettingsService::class);
         $this->siteSettingsFactory = $this->createMock(SiteSettingsFactory::class);
+        $this->writeGuard = $this->createMock(SiteSettingsWriteVerifierInterface::class);
 
         $GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS']['analytics']['apiBaseUrl'] = '';
         $GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS']['analytics']['verifySsl'] = '0';
@@ -71,6 +76,7 @@ protected function setUp(): void
             $this->cipherService,
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->writeGuard,
             new NullLogger(),
         );
     }
@@ -197,6 +203,37 @@ public function provisionIfNeededDoesNotWriteWhenResponseIsIncomplete(): void
         $this->subject->provisionIfNeeded($site, ['status' => 'active']);
     }
 
+    #[Test]
+    public function provisionIfNeededReturnsWhenWriteSettingsThrows(): void
+    {
+        $this->mockHandler->append(new Response(200, [], '{"apiKeyId":"new-key-uuid","apiKey":"new-api-key"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->siteSettingsService
+            ->method('writeSettings')
+            ->willThrowException(new SiteConfigurationWriteException('disk full', 1590487411));
+
+        $site = $this->buildSite('w-123', 'i-456', $this->encryptedSecret);
+        $this->subject->provisionIfNeeded($site, ['status' => 'active']);
+
+        $this->expectNotToPerformAssertions();
+    }
+
+    #[Test]
+    public function provisionIfNeededLogsErrorWhenApiKeyCouldNotBePersisted(): void
+    {
+        $this->mockHandler->append(new Response(200, [], '{"apiKeyId":"new-key-uuid","apiKey":"new-api-key"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->writeGuard
+            ->method('assertSettingsPersisted')
+            ->willThrowException(new AnalyticsApiException('Credentials could not be written.', 0));
+
+        $site = $this->buildSite('w-123', 'i-456', $this->encryptedSecret);
+        $this->subject->provisionIfNeeded($site, ['status' => 'active']);
+
+        // No exception thrown — the service returns silently and logs an error.
+        $this->expectNotToPerformAssertions();
+    }
+
     /** Helpers */
 
     private function buildSite(
diff --git a/Tests/Unit/Service/InstanceRegistrationServiceTest.php b/Tests/Unit/Service/InstanceRegistrationServiceTest.php
index 803b589..91ca692 100644
--- a/Tests/Unit/Service/InstanceRegistrationServiceTest.php
+++ b/Tests/Unit/Service/InstanceRegistrationServiceTest.php
@@ -18,6 +18,8 @@
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
 use T3G\Analytics\Service\InstanceRegistrationService;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
+use TYPO3\CMS\Core\Configuration\Exception\SiteConfigurationWriteException;
 use TYPO3\CMS\Core\Http\Client\GuzzleClientFactory;
 use TYPO3\CMS\Core\Http\RequestFactory;
 use TYPO3\CMS\Core\Http\Uri;
@@ -36,6 +38,7 @@ final class InstanceRegistrationServiceTest extends UnitTestCase
     private array $httpHistory = [];
     private SiteSettingsService&MockObject $siteSettingsService;
     private SiteSettingsFactory&MockObject $siteSettingsFactory;
+    private SiteSettingsWriteVerifierInterface&MockObject $writeGuard;
 
     private CipherService $cipherService;
     private InstanceRegistrationService $subject;
@@ -54,6 +57,7 @@ protected function setUp(): void
 
         $this->siteSettingsService = $this->createMock(SiteSettingsService::class);
         $this->siteSettingsFactory = $this->createMock(SiteSettingsFactory::class);
+        $this->writeGuard = $this->createMock(SiteSettingsWriteVerifierInterface::class);
         $this->cipherService = new CipherService();
 
         $GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS']['analytics']['apiBaseUrl'] = '';
@@ -70,6 +74,7 @@ protected function setUp(): void
             $this->cipherService,
             $this->siteSettingsService,
             $this->siteSettingsFactory,
+            $this->writeGuard,
             new NullLogger(),
         );
     }
@@ -168,6 +173,49 @@ public function registerStoresEmptySecretWhenApiResponseOmitsIt(): void
         $this->subject->register($this->buildSite(), 'user@example.com');
     }
 
+    #[Test]
+    public function registerThrowsBeforeApiCallWhenDirectoryIsNotWritable(): void
+    {
+        $this->writeGuard
+            ->method('assertDirectoryWritable')
+            ->willThrowException(new AnalyticsApiException('Cannot write to config/sites/main/.', 0));
+
+        $this->expectException(AnalyticsApiException::class);
+        $this->expectExceptionMessage('Cannot write to config/sites/main/.');
+
+        $this->subject->register($this->buildSite(), 'user@example.com');
+
+        self::assertCount(0, $this->httpHistory, 'API must not be called when directory is not writable.');
+    }
+
+    #[Test]
+    public function registerThrowsAnalyticsApiExceptionWhenWriteSettingsThrows(): void
+    {
+        $this->mockHandler->append(new Response(200, [], '{"websiteId":"w-123","instanceId":"i-456","instanceSecret":"s3cr3t"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->siteSettingsService
+            ->method('writeSettings')
+            ->willThrowException(new SiteConfigurationWriteException('disk full', 1590487411));
+
+        $this->expectException(AnalyticsApiException::class);
+
+        $this->subject->register($this->buildSite(), 'user@example.com');
+    }
+
+    #[Test]
+    public function registerThrowsWhenSettingsCouldNotBePersisted(): void
+    {
+        $this->mockHandler->append(new Response(200, [], '{"websiteId":"w-123","instanceId":"i-456","instanceSecret":"s3cr3t"}'));
+        $this->siteSettingsFactory->method('loadLocalSettings')->willReturn([]);
+        $this->writeGuard
+            ->method('assertSettingsPersisted')
+            ->willThrowException(new AnalyticsApiException('Credentials could not be written.', 0));
+
+        $this->expectException(AnalyticsApiException::class);
+
+        $this->subject->register($this->buildSite(), 'user@example.com');
+    }
+
     /** Helpers */
 
     private function buildSite(): Site
diff --git a/Tests/Unit/Service/SiteDataProviderTest.php b/Tests/Unit/Service/SiteDataProviderTest.php
index 987ced9..c6116c9 100644
--- a/Tests/Unit/Service/SiteDataProviderTest.php
+++ b/Tests/Unit/Service/SiteDataProviderTest.php
@@ -19,6 +19,7 @@
 use T3G\Analytics\Service\CipherService;
 use T3G\Analytics\Service\HmacSigner;
 use T3G\Analytics\Service\BackendPageAccessCheckerInterface;
+use T3G\Analytics\Service\SiteSettingsWriteVerifierInterface;
 use T3G\Analytics\Service\SiteDataProvider;
 use TYPO3\CMS\Backend\Routing\UriBuilder;
 use TYPO3\CMS\Core\Cache\Backend\TransientMemoryBackend;
@@ -103,6 +104,7 @@ protected function setUp(): void
             new NullLogger(),
             $this->createMock(SiteSettingsService::class),
             $this->createMock(SiteSettingsFactory::class),
+            $this->createMock(SiteSettingsWriteVerifierInterface::class),
         );
 
         $this->pageAccessChecker = $this->createMock(BackendPageAccessCheckerInterface::class);