Regarding the --auth-u error, please look at this post to see if it may apply in your case: ?auth-u automatically added to destination path - #2 by ts678
I do not believe you need to attempt a database repair because of the above error - it sounds more like a job configuration issue.
It may be running as a user with Administrative rights, but is it running Elevated? Task Manager Details tab may help you identify this for sure. (You may have to turn on the “Elevated” column - I don’t recall if I did that or if it is visible by default.)
I’m not familiar with this one, but there is a discussion about issue 1400 on Github: Failed to process path: xxx => Unexpected result count: 0, expected 1 · Issue #1400 · duplicati/duplicati · GitHub
Are you trying to back up Duplicati’s own database?
Are you using snapshots?