Hm, I was playing a bit more and changed to "Client" instead of "Player".
When I now try to jump in I receive "YOUR FLIGHT IS DELAYED TO START, PLEASE WAIT".
Which is interesting; because my mission starts 18:30:00, and my player group also starts 18:30:00. Might there be a bug in the start times ... I have, after all, been editing a previous mission where I originally started from land.