@qole: jap, found that already but thanks anyway.

• Removed encfs, checked repositories, reinstalled encfs.
• Checkt whether onfirstchroot-ext.rc is executable and contains correct code
• checkt users of groups
• restarted Debian several times

... no luck at all :-(

I've got a bad feeling about kernel-power47 beeing responsible on that although fuse is available. Maybe someone using this kernel version could verify?