Failed to create volume snapshot

Hi,

I've got an ABR 11.5 running on one of our servers to do full backup on the server's disk on a daily basis. Randomly maybe 1 out of 5 backup failed with this error below.

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Log Entry Details
--------------------
Type:          Error
Date and time: 15/04/2014 9:32:23 PM
Backup plan:   [None]
Task:          [None]
Code:          66,038(0x101F6)
Module:        1
Owner:         Administrator
Message:      
  Operation with partition '0-0' was terminated.
Details:
Failed to create volume snapshot.

    Error code: 0x70021
    $module = "C:\Program Files (x86)\Acronis\BackupAndRecovery\DiskBundle.dll"
    Tag: 0xA5695862AAF8E674
Failed to start creating the volume snapshot.

    Error code: 0x10C442
    Tag: 0x14181C22EF45B077
Unknown Win32 error

    Error code: 0xFFF0
    code = 2,147,754,771 (0x80042313)
    Tag: 0xBD28FDBD64EDB8BC
Additional info:
--------------------
Error code: 502
Module: 1
LineInfo: a164035b3ff39287
Fields:  $module : C:\Program Files (x86)\Acronis\BackupAndRecovery\DiskBundle.dll
Message: Operation with partition '0-0' was terminated.
Details:
Failed to create volume snapshot.

    Error code: 0x70021
    $module = "C:\Program Files (x86)\Acronis\BackupAndRecovery\DiskBundle.dll"
    Tag: 0xA5695862AAF8E674
Failed to start creating the volume snapshot.

    Error code: 0x10C442
    Tag: 0x14181C22EF45B077
Unknown Win32 error

    Error code: 0xFFF0
    code = 2,147,754,771 (0x80042313)
    Tag: 0xBD28FDBD64EDB8BC
--------------------
Error code: 33
Module: 7
LineInfo: a5695862aaf8e674
Fields:
Message: Failed to create volume snapshot.
--------------------
Error code: 50242
Module: 16
LineInfo: 14181c22ef45b077
Fields:
Message: Failed to start creating the volume snapshot.
--------------------
Error code: 65520
Module: 0
LineInfo: bd28fdbd64edb8bc
Fields:  code : 2147754771
Message: Unknown Win32 error
--------------------
Acronis Knowledge Base: http://kb.acronis.com/errorcode/

Event code: 0x000101F6+0x00070021+0x0010C442+0x0000FFF0

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Microsoft VSS is used instead of Acronis VSS. I just wonder why it is failing when everything else is good.

What seems to be the problem here or the cause perhaps?

Thanks,

Karl