Sorry for the late late late answer !!
You are the second person mentionning this, I believe it appears when there is a disconnexion between the two players. Steam connexion might manage to make you both reconnect but the game does not know how to handle this.

The easiest way to recover is to go back to the Main Menu and restart the session with your friend (I hope you have found this, and are not stuck since almost two months ago omg).

I will try to fix this for the next update (no date, but there it will happen).

I hope you still like our game :D

Any updates on this issue? My fiancee and I are experiencing the same problem. As the host, the game loads for me, but she gets stuck with the "waiting for all players to connect" text. Restarting the session doesn't seem to fix the issue, either.