Hi,
the Jabber UID is displayed instead of the username, when the directory lookup fails.
What kind of directory integration do you use? LDAP, UDS? Is it configured properly? Check the status of the directory connection under Help>Show Connection Status, when this occurs.
Is this also happening in the contact list? The information on users in your contact list should be cached locally, so there might also be a problem with that.
Matthias