this is related to the user post here:
it seems to happen in backup_execute() in backuplib.php
obviously for some reason, backup_delete_old_data() is failing and not deleting the directories inside directories completely - I haven't looked closely - but is this a bug in backup_delete_old_dirs() ? - the user is using an older version of moodle so it's potentially fixed.....
I suggest we change the call in backup_execute:
... to use a debugging call instead of the error() function.. - the fact that it hasn't deleted all the old dirs in temp doesn't actually affect the backup itself, which should succeed fine, and simply displaying a warning should suffice IMO