You will see this error when you run DPM 2010 to backup CSV. On the DPM 2010 console, the backup is successful and can restore VHD backup. However on the Event Viewer, you will see Event ID 1584 is logged.
In my case the NetApp FAS used for SAN storage and the reason is did not purchased Snapmanager to support hardware requestor for VSS.
To solve this issue, you need to enable serialization backup of CSV node.
On DPM server, modify the registry (change data from 3 to 1)
Key : HKLM\Software\Microsoft\Microsoft Data Protection Manager\2.0\Configuration\MaxAllowedParallelBackups
Value: Microsoft Hyper-V