Task Scheduler Logon failfure

I schedule a daily backup task from Ntbackup.
It doesn´t work and the Task scheduler log show this
message:

"BU Semana 1 Incremental.job" (ntbackup.exe) 03/01/2003
9:55:00 ** ERROR **
The attempt to log on to the account associated
with the task failed, therefore, the task did not run.
The specific error is:
0x80070569: Logon failure: the user has not been
granted the requested logon type at this computer.
Verify that the task's Run-as name and password
are valid and try again.

The task "Run as" a Backup Opertator user, the same which
the computer is logged on. It is a Windows 2000 Domain
Controller.

If I run manually the task from the Task Scheduler it
doesn´t work, but if I open the task properties, write the
password in "Set password" and run the task manually it
works. And the next time the task runs automatically ¡it
doesn´t work!.

Thanks a lot
:confused: