There could be many reasons that may cause a 156 snapshot error. Please can you give more details on your setup and what kind of backup job you are running? Is it a regular client backup or maybe a VCB backup etc?
VEN , you are still not giving enough info. What is the policy type of the failing backup? What is the client O/S? NetBackup will by default try to use VSP for open file backups on Windows. If the client is W2003 or W2008, you should change the config on the Master server Host Properties -> Client Attributes, add Windows client name, and select VSS.
See this TechNote for more info (although the status code is not the same): http://seer.support.veritas.com/docs/317730.htm
By default any new client added to a backup policy will use a Windows Open File Backup (WOFB) method for MS-Windows-NT policies. The default snapshot for WOFB uses the Veritas Snapshot Provider (VSP).
To change the snapshot provider from Veritas Snapshot Provider (VSP) to Microsoft Volume Shadow Copy Service (VSS).
1. In the NetBackup Administration console go to Host Properties, Master Server.
2. Open the Master Server.
3. Go to Client Attributes.
4. Add the name of the client and be sure to use the same case sensitivity as that used in the policy.
Note: If the client name is LEVEL in the policy do not add the client name LeVeL in host properties.
NetBackup may treat LEVEL and LeVeL as two different clients.
5. Once the client is added, close the client add dialog box and select (highlight) the client.
6. Select the tab for Windows Open File Backup.
7. Select Use Microsoft Volume Shadow Copy Service (VSS).
8. Click OK to save the change.
Note: A restart of services or daemons is not required for this action.