So I searched the forums, and I was told writing the serial in all small letters without the -'s would fix the problem.
So I got in to a lobby, once. But that didn't lead anywhere but everyone biching about Multiplayer on the chat, and the game never started.
I had to go back out to the server browser. And after painfully waiting for that to actually work after a few "invalid parameter" messages, and all servers showing 109 ping, and 5 seconds for it to react to my clicking the server, I got the "Serial Code Already in Use" message again.
So... what now? Am I supposed to sit and wait for days for you to fix a game I spent €50 buying?
(edit, bought on Steam)