I recently reinstalled the game and everything with it.
T3A has been working for me. I played some 1v1s and a 2v2 today. The 2v2 was laggy because of me but the 1v1s were fine. I then changed the options.ini (as instructed to do on this site to help with connection issues) and I was then unable to connect. Reverted the changed option.ini and I was able to connect on-host and off-host. This has been with 5 players so far. Hopefully it continues.
Did you guys change anything over the last half a year or so?