All issues I've seen with system_state/shadow copy backups have been fixed by following a few simple procedures:
1. Ensure that the client is set up to use VSS, not VSP. Don't even install VSP.
2. Uninstall and re-install the client
3. Ensure that you have at least 300MB of free space on the system drive
4. Install the latest hotfixes for VSS. (sysadmins typically don't do this).
http://support.microsoft.com/kb/940349
Very rarely we still have issues and then have to follow a Microsoft KB article that resolves this (
http://support.microsoft.com/kb/940184)
Always remember that NetBackup is the VICTIM of VSS issues, not the culprit.
.../Ed