Meanwhile playing with a friend, we were doing some expeditions and encountered a very interesting behavior:
If one player dies and awaits to be resurrected and the other player dies a bit after, the first player then has the posability to spam the revive button. Sometimes it works and you can respawn but it just so happened to be at the exact same time as my friend died. What happened was that the expedition failed but I respawned non the less inside the expedition. My friend on the other hand was back in Stormfall. The stage was completely black but I could see the map and the HUD. Then I began to move and fell through the stage and landed at the "bottom". Sadly I was not able to record the fall but I do have footage of the "bottom". Felt more like the shadow realm but whatever :D

Link to the video on imgur: https://imgur.com/gallery/iT3AwCr

Hope this helps you fix the issue.
Regards,
Dice