LCS error


we are using LCS 2005 our one of the user is getting attached error any suggestions.
Who is Participating?
Is this a problem for only one user?
What OS and Service Pack is he using? On XP there used to be such a problem because of Kerberos: 
Does the same user have this problem when he tries to log in from another machine?
In which format does he enter his credentials?
Did this ever work before?

Check out the steps in the following article:
Common Issues with Sign-in and Discovery 
Make sure of the following
He is using windows messenger 5.1 or less
He is entering correct domain username and password
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