Backup failed (w2) error

I just got NOBU and it has not been able to back up.  Every time it tries it gets to the 9 GB mark and stops.  The "Backup Stopped" message pops up.  The activity log has the red x on the icon and "Backup failed (w2)" message.  No explanation.  It stops at the same point everytime.  If I can't get it to work I will need my money back.