Problem: Joining a game causes players to "leave" after 10-15 seconds and prompts the game to notify that you're playing alone.

So I've sent an inquiry to Deep Silver about it and they sent me a reply. The fix was so simple (and effective; I didn't have any problems with my co-op sessions after): disable your WiFi.

Of course you can just disable it through your admin panel but for those who aren't tech-savvy (or if they don't have admin rights) here's a quick guide about how to do just that.