1) Check with Ntbackup if the problem persists.
2) Reboot the server once and check the result.
3) Perform single volume snap at at time backup and check the result.
http://support.veritas.com/docs/269591
If nothing helps, Apply the Sp1 and check the result.
VERITAS Backup Exec (tm) 10.0 Service Pack 1 (SP1) for Windows Servers revision 5484 :
http://support.veritas.com/docs/275248