A parallel backup operation may fail when the operation is performed by Volume Shadow Copy Service (VSS)
When Volume Shadow Copy Service (VSS) performs a parallel backup operation on a computer that is running Windows Server 2003 Service Pack 2 (SP2), Windows Vista, or Windows Server 2008, the operation may fail. When this issue occurs, the VSS writers are in a “corrupted” state, and the VSS writers return the following error message:
For example, you may encounter this issue when you use System Center Data Protection Manager to perform a parallel backup operation.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB975928.