I think it's the issue that's already fixed in xna-cncnet-client 2.6.2.4, in 02e82aba. I compiled the latest version of the client and it allows me to join a lan game again. The version of the xna-cncnet-client that's bundled with the 6.3 update is version 2.6.0.5, which is a bit behind. Hope this will get released soon!