After applying the November Windows Updates, we found that DPM was no longer able to back up our SQL databases. We figured it was one of the updates (since machines we hadn’t patched yet were still working). I little searching around found this great blog (http://blogs.blackmarble.co.uk/blogs/adawson/post/2014/11/17/DPM-2012-R2-SQL-Backups-Failing-After-November-2014-OS-Updates.aspx) that documented the same symptoms, but they’d already figured out which patch was the culprit – KB3000853.
We pulled that patch and backups resumed. If you find yourself in the same boat you can run wusa /uninstall /kb:3000853 /quiet /norestart to pull it.
Hope this helps!