It isn't just a patch for multiplayer. It fixes things in single player, too.
Unfortunately, this means that your previous saved games can no longer be used. You must start the game over. Loading older saved games will cause the behavior you described.
Sucks, I know. But without the patch, you can't play multiplayer.
All of these things are explained in the
Mini-FAQ post I mentioned to you earlier. I wish more people would read it.