Thanks for your answers.

camera-ui (and the whole camera thing) is the one with stable CSSU or if CSSU didn't change it, it should be the original one from Nokia. I didn't touch it.

The permissions of the block devices /dev/mapper/mydocs_luks and /dev/loop4 differ. But it doesn't matter if I mount /dev/dm-1, which is the same major/minor than /dev/mapper/mydocs_luks but with the same owner/group/permissions as /dev/loop4. (Or if I change the permissions on /dev/mapper/mydocs_luks)

No luck so far.