happened to me as well.
6 different customers - 6 different environments. the media server on all of them is the domain controller.
all of them without any difference failed with the same message.
bug in the sp3.
i just excluded this folder from all backups to get success but it's a little bit strange that symantec released this SP3 after such a long wait (6 months) and such a feature will not be already implemented into the SP.
also i was very disapointed that still no solution for 2012 as per multiple server backup from one job - thought i was already going to upgrade all of my customers but apparently symantec would like them to just leave her for other companies that have simple solutions.