[Serusers] Instant Messaging cannot work with Windows Messenger 5.0

Hi, I have two PCs running Windows Messenger 5.0 and SER as registrar. I am able to get Voice, Video, App Sharing, Whiteboard, Presence (peer to peer only) working. However, I cannot get Instant Messaging to work. When I look at my Ethereal capture, I see that with 5.0, INVITE is used to start an IM session. The other client responds with TRYING, then OK. However, it also sends a BYE immediately after sending the OK. Subsequently when the originating client sends the MESSAGE, the other client responds with an Error because "Call Leg does not exist". Has anyone encountered this before? I've seen this problem being mentioned in Google searches, but no answer so far. Regards, KokMeng Loh