I had some problems trying to do coop with my friend. We could invite each other, but if I was invited I got a permanent loading screen and my friend got a freeze. If I invited my friend I got into the game, but they were stuck on the loading screen. However, they got a health bar when their meat bro took damage.

Unfortunately I figured out how to open the log after this, but I made my friend go through the tutorial level and after that everything worked fine. Might work for other people as a workaround.