The following are the possible causes for this problem:
Problems while accessing the LDAP server, such as the LDAP server is not running, or a provisioning error, such as a schema violation, has occurred.
End user not found.
Invalid Instant Messenger session.
Where to get diagnostic information:
Instant Messaging server, Identity authentication, and LDAP log files.
In deployments using Sun Java System Access Manager, ensure that the user entries in your Directory contain the iplanet-am-managed-person objectclass. The Instant Messaging server uses this object class when it searches for valid users in an Access Manager deployment. For more information about this object class and how Access Manager uses it, refer to the Sun Java System Access Manager documentation.