How far are you and your friend into the game's storyline - does the game indicate about 93% of progression for any of you? Starting from around 93%, all the quests (including side quests) need to be completed in single-player only by design. Once you complete the main campaign to 100%, co-op will become unlocked again. If, however, your quest progression is below 93% and you still cannot join your friend(s) Is this true or not??
Me and my friend have done every suggestion listed above to fix the coop connection issues, even suggestions given to both of us by techland support as well, and nothing has worked. This is unacceptable seeing as though techland has promised in this very forum, that they will address this issue. This is clearly an internal issue within their servers and any suggestion that its not is a bullshit excuse to fix their own mistakes. FIX THE chupacabra ISSUE.