I've run in to this issue. My game was slowing down so I wanted to reboot my laptop. When I restarted the game and loaded it, my wife couldn't join. It would go to the loading screen then drop her back to the main menu.
We tried a combination of the following actions.
- Restart game.
- Reboot Laptops.
- Load a different save game.
- Join the Host.
- Host invites.
Nothing specific seemed to fix it. We just went back and forth restarting and eventually I think it was my invite that got it working again. A previous invite did not work. I knew when I saw her game loading very slow that it was going to work.
My only theory at the time was to attempt to connect enough times that the bug would get tired of us trying and give in. Think it took a total of 5-10 minutes (including reboot/game restarts) to get it working. Haven't had the issue pop up again so far.
Wish there was a pattern I could provide to help you out.
