XP Scheduler Errpr

Hello,
Every night we run a backup on all the computers using xp’s backup utility and the windows scheduler. every week or two the job stops running and we get the following error

The attempt to retrieve account information for the specified task
failed; therefore, the task did not run. Either an error occurred, or no
account information existed for the task.
The specific error is:
0x8004130f: No account information could be found in the Task
Scheduler security database for the task indicated.”