Made my day!
Before I tried to install duplicati with the .deb-file from their website - but something is broken since omv 5.?
My problem is, that the docker user has no file access rights for i.e. the home-dirs of other users.
So I used the root user uid+gid and that worked. But this is probably not the best method.
Is there another way to give the docker user access to ALL files that shall be backuped?