Can you check if Open file backups are not desired for the client, disable them. This will decrease the resources used by the client during backup and resolve any Status 156 errors. The default NetBackup install options are to have open file backups enabled.
To confirm if VSS or VSP open file backups are activated:
1. Within the NetBackup Administration Console, expand Host Properties in the hand pane
2. Within Host Properties, click Master Server and then the double-click the name of the Master Server in the right pane
3. Click on the Client Attributes section
4. If the client in question is listed here, click on the client name. If it is not listed, add and click on the client name.
5. Click the Windows Open File Backup tab
6. Ensure that Enable Windows Open File Backups for this client is not selected if open file backups are not required
If open file backup functionality is needed, the most common cause for snapshot errors is cache file configuration. The steps on changing this depend on whether VSP or VSS is used as the Snapshot Provider.
I hope the policy being used to take this backup has type "MS-WINDOWS" and not "STANDARD". Please confrim.
I could see some related problems with bpbrm error code 156. it looks to be a issue related to the volume thats mounted on the client system. Try rebooting client if possible and fire backup.
Which NBU version you are using?
I recommend opening a service request with Symantec on this problem.
NOTE: Please make a note that this is not a BMR process problem. Something is going wrong in NBU backup process while taking shadow copy components backup.
This comment may help handling service request better.