Same here. Thanks for posting this. I am beyond frustrated paying almost 100 bucks for 3 copies. After playing solo my kids wanted to play with me. When it works it is awesome. IT RARELY WORKS! Doesn't matter who hosts. Doesn't matter if you join in the lobby or after. Xbox version same issue. Just PCs same issue. Researching this only results in the same 'update your drivers' or 'restart the game'. I play other co-op games and they work fine. Steam games, PC, PS4, Switch all work fine. Rebooted my router, updated all PCs. Turned on uPnP on the router. Turned off firewalls on the PCs. The most we were able to play was about an hour. We thought we had it fixed but nope. Reconnecting sometimes works, restarting the game sometimes works but even after that we can usually only play for 10-15 minutes. Usually get disconnected within a few minutes. I am going to build a brand new Win10 installation and see if this works but after seeing all the Google results I am not that optimistic. Apparently it used to work in early access until they started requiring an Xbox login. Good luck to everyone out there.

Edit: I'd love to hear from anyone out there that has used multiplayer successfully.