W2K Scheduled Backup Does Not Run

This has happened on two installations recently. Windows 2000 server SP2 or SP1 using the native backup program and scheduler. Backup ran the first time but has not run since. We have deleted the job and set it again to no avail. Manual backups are fine. If we schedule it in the near future and watch the process, it is obvious that the schedule starts the job but reports an invalid command line parameter after the backup starts. We have been unable to find anything on Technet regarding this problem. Has anyone seen this problem before and found a solution BESIDES buying a real backup program? We’re stuck!