Game crashes when I try to play with a friend

So whenever I try to play with one of my friends, my game crashes and pulls up a bug spelt error report. I can play games by myself, but whenever I try to play with him, it kicks me when the loading screen comes up. I've tried to reconnect, I've tried restarting my client and even my computer, but none of that works. I'm worried that I'm going to start getting game penalties. Anyone know how to fix it, I'm on Mac OSx.

Edit: I have been able to play with him in the past, this has only recently started happening, I'll update this every so often, I can't reply to comments as my post button does nothing upon pushing it when trying to reply or post on another thread...