Good question, it should not delete things then, but the error message indicates that this is what is happening. You should see a message like Deleting 1 remote filesets in the log for the backup.
My guess is that somehow a remote volume is marked as deleted incorrectly, and then all the database queries pick up this problem.
If you can make a bugreport for the database, I can have a look and see if I can fix it, but it will be a week or two before I can look.