Hyperv Windows 7 Guest VM fails to backup

We're running into an issue with Hyper-V backup.

Trying to backup a windows 7 guest the backup fails with the following error

[Normal] From: OB2BAR_VSSBAR@ndhv01.xxx.com "HyperV"  Time: 7/18/2014 11:45:32 AM
    Exiting Shadow Copy Set creation phase.

[Major] From: OB2BAR_VSSBAR@ndhv01.xxxxx.com "HyperV"  Time: 7/18/2014 11:45:34 AM
[145:575]     Writer 'Microsoft Hyper-V VSS Writer' failed to prepare files for backup:
           Reported state:    VSS_WS_FAILED_AT_PREPARE_SNAPSHOT
           Expected state:    VSS_WS_WAITING_FOR_BACKUP_COMPLETE
           Failure code:        VSS_E_WRITERERROR_NONRETRYABLE

[Major] From: OB2BAR_VSSBAR@ndhv01.xxxx.com "HyperV"  Time: 7/18/2014 11:45:34 AM
    Cannot perform backup of:
        '/Microsoft Hyper-V VSS Writer/Virtual Machines/Online\MRP',
    which contains data in:
        C:\ClusterStorage\VM02\MRP\Virtual Machines\16134A71-D987-48EB-A738-44EA749A31E1.xml
        C:\ClusterStorage\VM02\MRP\MRP.vhdx
        C:\ClusterStorage\VM02\MRP\MRP-AutoRecovery.avhdx
        C:\ClusterStorage\VM02\MRP\Snapshots\EA67B48A-64BD-4415-97DA-B8B68048C005.xml

[Major] From: OB2BAR_VSSBAR@ndhv01.xxxx.com "HyperV"  Time: 7/18/2014 11:45:34 AM
    There is no data to be backed up.

[Major] From: OB2BAR_VSSBAR@ndhv01.xxxx.com "HyperV"  Time: 7/18/2014 11:46:10 AM
    Backup failed.

All other VMs backup ( windows 2008 or windows 2012 ) went fine

Any idea ?

thanks