NTbackup doesn't complete, leaves a blank log file, VSS errors in application log

I have a Windows 2003 server that is running a semi-custom backup task. The scheduled task calls NTbackup with a few switches depending on whether it is a full or incremental backup. Most of the time, the NTbackup completes fine, and the wrapper then appends the NTbackup log into its own log before adding a few final comments and completing.

The problem I am having is that sometimes, NTbackup seems to just... blank out. It always completes backup of the C: and E: drives, but then it will start the system state and not add any more messages into the event log saying it completed that. And the NTbackup log is left empty, since it doesn't write anything to the log until all the backup tasks are complete. This is causing the wrapper to append no text into its own log. That causes problems for us because we read the information out of that log to determine whether backups are failing. The wrapper task also reports that it is completing normally in the event log.

Anyone ever seen a case where system state doesn't complete consistently? To be clear, the server is not logging any error messages anywhere. It's just not seeming to complete or log anything.

EDIT: I have discovered that this sometimes happens after it begins the backup of the E: drive, so it's not specific to the systemstate portion, as it has failed on the E: drive with or without systemstate included in the backup.

UPDATE 9/26/13: I have found that VSS is posting the following two errors to the application log:

Source: VSS
Event ID: 8193
Volume Shadow Copy Service error: Unexpected error calling routine
IMultiInterfaceEventControl::GetSubscriptions.  hr = 0x800706bf.

Source: VSS
Event ID: 12291
Volume Shadow Copy Service error: Error on creating/using the COM+
Writers publisher interface: BackupShutdown [0x8000ffff].

I've researched these some, and I went through the suggested troubleshooting. The COM+ Event System server is running and set to automatic startup. The Volume Shadow Copy server is set to manual startup. I ran vssadmin list writers with admin permission and all five writers listed showed their state as stable and last error as no error. I've read some about changing group policy permissions for the account running the backups, but I'm not clear on how or what to do. And this is not happening consistently, as I said. Sometimes the backup runs fine, sometimes it fail to complete with the errors above, and sometimes it completes with those errors. In the former case, the errors show up a few minutes after the backup just stops, and in the latter, the errors show up within milliseconds of the backup's completion.