by Eloy Lafuente (stronk7).
Hi Richard,
the first thing I'd try is to enable debugging in your site in order to get more details about which exact error you're getting there.
With that it may be possible to understand what's going. Still I'm not sure how both things (the failures in the adhoc task and the backups) can be related... but one thing at a time.
Hope debugging gives you some clue!
Ciao