JSI Tip 0933. How do I abort a scheduled NTBACKUP that is not responding to Taskmgr?.

The cause of your problem is probably your failure to use the /interactive switch with the AT command and NTBACKUP is trying to issue a message.

In tip 656, I described how to kill a process using KILL from the Resource Kit.

If you don't have the Resource Kit, you can:

AT HH:MM /interactive taskmgr

where HH:MM is two minutes from now. When Taskmgr starts, it will do so under the same account context as the scheduled NTBACKUP. It should then be able to kill the process.