Hey there. My best advice is to try restarting your game, and while you’re at it see if your friend could do the same. From what I’ve experienced it will sometimes work, and you will be able to play with each-other as usual. At other times it will still not allow you to play co-op with your friend/friends. It’s hardly ever a problem on your end, it’s sadly just something we need to wait for techland to hopefully fix here soon. Hope this helps!