Had to do some weird wizardry to play online: As host:

1. Try to host normally
2. the other player tries to join
3. host stops hosting
4. client gets error, goes back to main menu
5. host starts a "singleplayer" game, loads into the game
6. client tries to join again
7. host gets a notification and accepts join request

We've also experienced some disconnections, after that we had to both restart the game and perform the co-op ritual again