"encrypted folder could not be mounted because the mount point is not empty"

[anodynos]

I always get "The encrypted folder could not be mounted because the mount point is not empty:", while:

• the mount point IS indeed empty & all users have access - I can mount both as encfs /mnt/DropBox/Dropbox/_boxed /mnt/boxed and sudo encfs --public /mnt/DropBox/Dropbox/_boxed /mnt/boxed
• I get the same message either when I use the right password, or a wrong one.
• CLI encfs correctly reports wrong password

I use encfs 1.7.4, fuse 2.8.6 & Kubuntu 12.04.4 (kernel 3.2.0-61)