Just started having an issue connecting to servers.

When trying to load to any large server the game closes and I end up getting this error at the bottom of my browser. "Game disconnected: Client closed down connection. Check your network connection and make sure that port 3659 (UDP) is open." Why would I have to open any port? I haven't changed anything with my network. Seems to be an issue with the game because I'm able to connect to empty servers and a server with one person in it.