We have had TSM running fine for several months.  Starting last Thursday, the 
backups on one of the clients started to fail.  Nothing has changed recently on 
the server or client machines.  Two other clients that are the same as the 
failing machine are continuing to work fine.  I have stopped and restarted the 
"dsmc sched" process, but the problem continues.

Does anyone have any ideas what may be causing this?

The error message we are getting in the administrative log is as follows:
  ANR0423W
  Session 3289 for administrator ÿÿÿÿÿUT ( ) refused - administrator name not 
registered. (SESSION: 3289)

The TSM server is as follows:
        AIX 5.1 ML 5
        TSM Client 5.2.2.14
        
The TSM client is as follows:
        AIX 5.1 ML 5
        TSM Server 5.2.2.14