Some domain's aren't backed up
Haven't seen this behaviour myself, but 3 people reported it on the forum:
https://www.howtoforge.com/community/threads/i-get-some-errors-after-upgrading-to-ispconfig-3-2.85384/
https://www.howtoforge.com/community/threads/backup-log-errors.85478/
We will have to do some more digging to find the problem...