V-79-10000-11217 - VSS Snapshot Error

BackupExec 2012 Rev. 1798 64 Bit with Service Pack 3 running on Microsoft Windows Server 2008 R2 Standard, backing up a Windows 2008 file server and Active Directory domain controller to LTO-5. The last two daily runs complained:

Backups of the C: volume have run fine for several weeks before. Backups of other volumes on the same server show no problem. The event log of the file server has these error entries:

Protokollname: System
Quelle:        volsnap
Datum:         28.01.2014 22:49:20
Ereignis-ID:   8
Ebene:         Fehler

The time limit for the erase and storage write operation for volume "C:" has been exceeded waiting for a write operation release.

Protokollname: Application
Quelle:        VSS
Datum:         28.01.2014 22:49:50
Ereignis-ID:   12340
Ebene:         Fehler

Volume shadow copy warning: VSS has waited more than 40 seconds for all volumes to be emptied. This caused a time out for the volume "\\?\Volume{01f4797e-e077-11e0-8533-806e6f6e6963}\" to occurr while waiting for the release write phase. This problem can perhaps be solved by performing the operation at a time with less disk activity. 

Operation:
   Asynchronous operation in progress

Context:
   Current state: flush-and-hold writes
   Volume name: \\?\Volume{01f4797e-e077-11e0-8533-806e6f6e6963}\

Protokollname: Application
Quelle:        VSS
Datum:         28.01.2014 22:49:54
Ereignis-ID:   12310
Ebene:         Fehler

Volume shadow copy service error: The shadow copy cannot be assured - operation exceeded time limit. Error context: DeviceIoControl(\\?\Volume{01f4797e-e077-11e0-8533-806e6f6e6963} - 0000000000000128,0x0053c010,00000000002C1810,0,00000000002C2820,4096,[0]). 

Operation:
   Shadow copies are being transferred

Context:
   Execution context: System Provider

Protokollname: Application
Quelle:        VSS
Datum:         28.01.2014 22:49:54
Ereignis-ID:   12298
Ebene:         Fehler

Volume shadow copy service error: The I/O write operations cannot be stored on volume "\\?\Volume{01f4797e-e077-11e0-8533-806e6f6e6963}\" during the shadow copy creation interval. The volume index in the shadow copy set is 0. Error details: Open[0x00000000, Operation completed successfully.
], Flush[0x00000000, Operation completed successfully.
], Release[0x00000000, Operation completed successfully.
], Execute[0x80042314, The shadow copy provider has exceeded the time limit while waiting for the write operation on the volume from which a shadow copy is created. A possible cause for this is high activity on the volume caused by an application or a system service. Repeat the operation later when the volume is not so busy.
]. 

Operation:
   Asynchronous operation in progress

Context:
   Current state: DoSnapshotSet

The backup runs at night when no one is accessing the server.

"vssadmin list writers" shows all writers as "stable/no error".

I searched the forum for the error code but found nothing that appeared to fit this case.

Any hints on troubleshooting this are welcome.