As far as I know, GoBack uses a special disk format (I guess it probably stores the restore information somewhere in hidden areas of the disk?)
avast!, on the other hand, is an ordinary program - the update/installation just places/updates a few avast! files on the disk - i.e. nothing that thousands of other programs wouldn't do. So, I can't see any way how avast! can interfere with GoBack. OK, theoretically (and it's pure speculation from me because I really have no idea how GoBack works exactly) there could be a conflict between avast! (disk access) drivers and GoBack (again, don't know how "deep" it works) - but such a conflict would be permanent, not something that appears during avast! program update only.
So, for me, it's simply a GoBack problem - because I really don't see any way avast! could cause this.