Backup stops at 20%

Acronis Backup & Recovery 11.5 (build 11.5.37687)

Backups started failing last week after 2 months of successful backups. The program stalls at 20% and the last entry in the log is "Locking Partition 0-0". When this happens you can not stop the backup through conventional means (must stop services). I thought maybe VSS was the problem so I changed it to use Microsoft and then it stalls at 9% and while I can stop it normally it still won't get any further.

I have tried repairing the install with no success.

Anyone else have this issue and a resolution? Not sure where to go next.

This is on Windows Server 2012.

Thanks.