Yeah, if you have a no-cd patch you'll get that error all the time in multiplayer. I heard some things about an elusive no-cd patch that didn't cause that error, but I could never find it.
So, yeah. If you want to play multiplayer, you'll need to either find the legendary working no-cd patch, or get a CD. I'm not sure if copied CDs will work or not. But there's your answer.