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When you schedule NTBackup, it randomly does not backup anything. You will not find any errors in the Backup log file, and you will not find any errors in the Scheduled Tasks log file. The task may still show status "Running" in Scheduled Tasks folder, but you can verify that it does not backup anything, and it will never finish.
If you look in Task Manager, you will find one or more (depending on how many times it has failed) ntbackup.exe processes, and if you try to open the GUI (Start->Programs->Accessories->System Tools->Backup), it will not start. Furthermore, if you kill all the ntbackup.exe processes from Task Manager, the GUI will work, but the backup will still fail randomly when scheduled.
A work-around is to make sure that nobody is logged on to the server when the backup is scheduled to run. Remote Desktop logons (connected or disconnected) will also cause NTBackup to randomly fail.