I removed everything from the steam folder and I'm doing a fresh install of the game. I'll edit this post again when it finishes and I try again.
EDIT NUMBER FINALLY: After who knows how many months, I don't have the error and can continue co-op shenanigans with my friend. Thank you so much for your help with all this. Glad I finally came on here and asked.
Last edited by wolfpein; 25/11/14 11:33 AM.