DP - Unable to close backup session - "failed to indicate success and complete backup"

Hi Guru's

A HyperV backup has started to fail with the error: -

[Major] From: OB2BAR_VSSBAR@mbhyperv4 "HyperV"  Time: 14/08/2013 03:07:49
    Failed to indicate success and complete backup.

[Major] From: OB2BAR_VSSBAR@mbhyperv4 "HyperV"  Time: 14/08/2013 03:08:26
    Backup failed

The session then hangs and I have to abort it. (takes 20mins)

It backups up the VM ok as it says 23GB backed up, but DP is unable to close the session correctly.

Thanks for any help.