The thing is that the Link protocol takes care of all the network setup and detection. We just get a notification when somebody wants to join the session and let others know we'd like to join one. We don't do much aside from that.
Have you checked if Link works correctly in other apps? Have you contacted Ableton about this?
Check your ASIO settings.
Link only works with ASIO.
Link is a network protocol. How does choice of a sound card affect network setup and detection?