I'm having to re-enter my serial key everytime I go into the multiplayer menu. Even if I enter it, go back to singleplayer, then hit multiplayer. Not sure if I'm missing something simple or not, but it's getting really frustrating.

I have this Problem, too. Plus if I want to join a Game it says "Code already in use"... (I have the Original Version here, of course..)

So after a bit of a search, I found this -

http://www.gamesas.com/forums/viewtopic.php?f=40&t=11741

I can confirm that I tried it and it fixed it for me.