This is how one guy solved it (Do you have 2 firewalls running when you play the game?):
All it was (for me anyhow as others are experiencing major problems) was that I had both Windows and Bullguard Firewall on together. Newbie error on my part (on the older PCs it actually informed you). Anyhow, I turned off Bullguard and it works. Obvious compatibility issues having two firewalls on.
I became a regular contributor which was suppose to give me a slot? I've never had a problem with that until recently. Now I'm a paying customer and getting a message "Could not join this server: it's full." when I try and join the servers.