I’m not certain I can help, but let me give it a try. This seems to be the first case in the forum of this exact issue, and even generalizing it only found posts that weren’t really helpful, so this may take some search.
For ReadyNAS, did you mean NTFS? That itself has quite a complex access control list system.
In another topic, I think you said you use NFS. If so, the question of translation comes into play.
Can you think of anything that may have changed or happened in the lead-up to this problem?
Please supply some OS information and the Duplicati version.
Do other backups backup to /backup as well? Any others to Private Videos? What are the mount points?
If on Linux, possibly you start Duplicati with a /usr/bin command. If so, which command and as what user?
Can you (with Duplicati down, and you as same user Duplicati runs as) add a test file and delete it there?
It sure looks from the stack like a delete is failing, so trying to do a similar operation safely may be useful.