I'm having the exact same problem. Running WS2012e and I've had all my server folders in a drivepool for a while. Recently, however, one of the pool hard drives died (I replaced it without incident and since everything was duplicated -- in theory without loss). Since around the same time, client backups aren't working.
I moved client backups to one of the pool drive root folders and they worked again, and I was able to backup successfully. So I moved them back to the pool, and I can't back up client computers again. I'm running v2.0.0.320.