If you're having the serial code is already in used for multiplayer, do the following.

1. Go to your user folder (default: c:\Users\YOURNAME\)
2. Right click on your Saved Games folder and go to properties
3.Untick read only and apply
4. Start the game and enter your serial code where it ask you at login page under where you put your id and pass.
5. Done

But can you JOIN a server without that error popping up? I tried that last night cuz i read about it in another forum and it doesn't do crap. I quit the game and checked the folder and it's set to READ only again itself. Baffled here.

I'm not sure if this works for steam since i was using eadm, if you are using steam, try checking the integrity of game cache first and try the steps again.