...how is the backup being done?
Backup to disk and the backup to tape? Or are you duplicating that information to tape instead, which is how things should be done.
See if you are getting Event ID: 57612 and Event ID:57608 in the application log. If so, make the changes as mentioned in the technote below
Backup to disk only
I didn't get Event ID 57612 and Event ID:57608. But this but the problem still occurs.
...might be worth opening a support call with Symantec around this. If they help you find a solution to this, please post back with the details and close this off.