There are many underlying components in the operating system that may come into play during the backup process to cause the VSS error. This video offers a guideline on how to troubleshoot for errors related to Volume Shadow Copy.
This Know-How video discusses how to rebuild a single backup set created in version 6 if the client application has not been updated and still runs version 6 AhsayOBM / AhsayACB, but the backup server has already been upgraded from version 6 to ve