A problem came up very suddenly a couple of days ago. My buddy and I had been playing a game almost every day, then suddenly we'd get a disconnect pop up (with the vote thing) right after starting up a multiplayer game. I hadn't changed anything in the game or the router settings.
I did a deinstall/reinstall of the game, checked the router, UDP ports, game exe settings, IP address in options.ini, all per the instructions on this site. The damn thing just keeps disconnecting immediately after game start. I tried to play with another player online, and it did the same thing. What else should I check?