We have seen this behaviour on 2 of our clients SBS2011 servers (and our own)
We have ntoiced that at 20.00hrs a backup will fail even though we have not specified an 8pm (20.00) backup.
In the SBS2011 Console - Backup and Server Storage - restore server data from backup - in here it lists all the backups performed and in the 'Messages' box at the top there are entries for 20.00 backups failing even though no 20.00 hrs backup has been requested.
In addition I can find no entry in scheduled tasks for this backup time.
The only thing I can think of - is that when you first set-up the SBS2011 backup facility the DEFAULT backup time is 8pm (20.00hrs). This has of course been removed / changed and there is only one backup performed - in this instance 23.00 hrs.
Can anyone help on this please.