error503official

This is the thing, neither ArmA II or ArmA II Operation Arrowhead asked me for the serial key. Could this be the reason?

I'm with a friend at my house right now, we're trying to play Day Z together on a server. But when we both try to join the same server, it doesn't work. What happens is that one of us can join just fine, but when the other one tries to join, it kicks computer 2 from the server immediately. The weirdest thing, though, is that whenever computer 2 tries to join, a message on the server appears saying "Remember: Illegal copies may degrade" WTF?! We both have legal copies of ArmA II Complete Collection on Steam! What's going on? We're both using Play WithSIX to run the mod.