Black screen when joining private coop server

[riceygringo]

We have a private coop server running. For some reason every mission there's one or two people who joins the server but cannot see any flights. The rest of us can see their names pop up in the lobby but the person only sees a grey box in the center of their screen in the lobby.

 

All ports are correctly forwarded on the server and the mission runs fine for those who can join flights.