I followed some tips from this thread (I can't post links yet I guess, but search for member Corgi and their post, or search 'Can't back up apps and settings on my 1520').
First I deleted live lock screen and rebooted, and tried backing up again. But that didn't work. So then I changed the lock screen, unlocked the phone, and then changed the picture back to my old picture. And backup worked! Hope this helps!