I'm not sure if this is related to T4 only but I just did some test and changing my name on the forum with this character: ë prevent me from connecting to any lobby, even if I start a solo game (talking about zombies here, didn't try the rest). I took a quick look in the console and I saw that something said "are we logged in?"
Whenever I changed my name back to anything else without special character it worked. So I suggest to either take a look at this in the game or to prevent users from using names with special characters on the forum.