When you view the Scheduled Tasks folder, the backup task may show that it is running, but nothing has been backed up.
If you open Task Manager, the Processes tab shows that one or more NTBackup.exe processes are running, depending on how may times it unsuccessfully tried to run.
When you attempt to open the NTBackup GUI, it will not open until you stop all NTBackup.exe processes.
This behavior may occur if a remote user uses Remote Desktop to log on to the server, and remains logged on, or if a remote user uses Remote Desktop to log on to the server and exits Remote Desktop without logging off.
To prevent this behavior, issue a net pause netlogon and net session /delete /y prior to running ntbackup.exe and a net continue netlogon when it finishes.
NOTE: you can add the above commands in your backup batch file.