Error 3279 "Access is denied due to a password failure" while running job

I  have a job that creates backups on a tape media and it was running fine until 2 days ago when it started to fail. This is the message on the job history:

    The job failed.  The Job was invoked by User sa.  The last step to run was step 1 (MSPAPELERA).  The job was requested to start at step 1 (MSPAPELERA).

    Executed as user: DIAGW2K2\Administrator. Access is denied due to a password failure [SQLSTATE 42000] (Error 3279)  BACKUP DATABASE is terminating abnormally. [SQLSTATE 42000] (Error 3013)  Associated statement is not prepared [SQLSTATE HY007] (Error 0)  Access is denied due to a password failure [SQLSTATE 42000] (Error 3279)  VERIFY DATABASE is terminating abnormally. [SQLSTATE 42000] (Error 3013).  The step failed.

I changed the job's owner to sa and to DIAGW2K2\Administrator but I still get the same error. I'm not using any kind of passwords to protect the tape.

This is the SQL Statement of Step1:

BACKUP DATABASE [MSPAPELERA] TO  TAPE = N'\\.\Tape0' WITH  INIT ,  NOUNLOAD ,  NAME = N'MSPAPELERA backup CINTA',  NOSKIP ,  STATS = 10,  NOFORMAT DECLARE @i INT
select @i = position from msdb..backupset where database_name='MSPAPELERA'and type!='F' and backup_set_id=(select max(backup_set_id) from msdb..backupset where database_name='MSPAPELERA')
RESTORE VERIFYONLY FROM   TAPE = N'\\.\Tape0'  WITH FILE = @i


Magda