We have 2 comps with FAF installed. I found the exe for FAF (1.5.3614) and started it on both (not trying to connect through FAF lobby). I went to host a multiplayer game, and then on the other comp I tried to join the game via LAN (they are both on the same router). The hosted game showed up fine on the list of games to join, but when I tried to join, it just stayed with the "connecting" dialog box without joining the game. So the comps are talking to each other (as the game is listed on the joining comp), but can't connect to get into the game lobby. I tried using the autoport option and manually entering in port 6112, but no dice.
I started up the 3599 exe on both comps and tried the same thing. This time, they were able to connect just fine. So in short, I believe something has been done in the course of modifications to the game with 1.5.3614 which is preventing LAN games.