Here is the error output directly from an Event Viewer occurrence of the issue:
The specified volume has already reached its maximum number of shadow copies. The volume has been added to the maximum number of shadow copy sets. The specified volume was not added to the shadow copy set. Other possible reasons: There is not enough free disk space on the drive where the locked file is located. Delete temporary files, empty recycle bin, etc. For WinXP, VSS has the limitation that only one shadow volume can be created per drive at a time. There could be another software that is already using the shadow volume for the drive. Restart your computer and try again.