Check on the event viewer (system and application) for event id related with VSS errors and take note of them.
Check the services, look for the Microsoft shadow copy provider and volume shadow copy and restart both of them.
Open a command line to check the writers status.
1. Type the following command: VSSADMIN LIST WRITERS
2. Review the output and identify the Last Error failure status for the specific VSS Application Writer involved in the backup operation.
3. Ensure that the remaining writers are all showing:
State:  Stable
Last Error: No error
If there are no errors with the steps written before proceed with:
Go to my computer and right click on the C drive.
Select properties and a new window will be displayed.
Select the tab called shadow copies.
Once in there you’ll be able to see the snapshots enabled or disabled for every single drive unit.
If the snapshot is not for every single drive push the button enable for each one.
Now that the snapshots are enable go to settings and check the no limit option.
After doing these you will need to check in Netbackup what kind of snapshot client you are currently using. It’s detailed below.
Service pack version of your 2003 OS, Installing W2003 service pack 2 is known to resolve this issue.
Take note of all these results for further actions to be done.
On the other hand, going to Netbackup try these:
Open the Netbackup console.
Go to host properties and expand it.
Select master servers and double click on it. You’ll be able to see the master servers properties window.
At your left look for Client attributes and search for the client your having troubles with.
After selecting the client with the 156 error code go to the 3 tab called windows open file backup.
Once in there check which client snapshot you are using, VSP or VSS, it’s recommended to use VSS with the disable snapshot and continue option checked.
In case the first steps done here were unsuccessful I will suggest you to disable the snapshot options clearing the option enable windows open file backups for this client until you were able to solve this problem with Microsoft or your windows admin.
In case you'll need to open a case with Symantec, set you debug loggin in 5 for the client in question and after running the backup gather the bpfis and bpbkar logs.