I had this problem too. I'd log in, it'd ask me for my key, I'd enter it, play a little then switch back to single player... when I went back to multiplayer, it asked me for the key again!

This is a coding error, obviously... But... Here's how I fixed mine... Perhaps it'll work for you too...

When at the log in screen, you'll see a box for your key and a "Modify" button. DO NOT press that button. Just log in, let it ask you for the key with a popup.

After that, exit the game. Don't go back to single player mode, don't do anything. Just exit the game.

When you open the game and log in again, your key should be saved.

Cheers.

THANK YOU

I was having the same issue as well, but I still can't login either (invalid username/password). this is really pissing me off. At least I can still play single player.

I am having this same problem. I tried what you suggested and my serial will still not save.

Hmm. I'm not sure then... That's how I got mine to save. One of the many problems with PC software on release day though it seems... PC games in general just seem to have so many more problems.

When I entered mine, I loaded the game and went straight to multiplayer and continued with the steps above... I didn't go anywhere else. It saved my key once I did that.

Yep.

having the same issues.

When I also tried jumping in a lobby, it said serial code in use.

That has me worried.

I did that earlier and it didn't help at all. Still won't save my key.