Reported on Reddit:

So my friend and I are trying to play Co-op together and everytime I try and host a game it works fine up until he tries to join me. When he tries to join me my game freezes for a moment and immediately kicks me to the main menu. We've tried restarting the game, changing the amount of players, changing it to public and restarting our consoles and nothing seems to work. Does anyone else have this problem or has had this problem? If you were able to get around some help would be very appreciated.

Edit: The player has confirmed that this also happens if they are the client rather than the host.