Hi Heiko and Jon,
I am not associated with Laminar Research: just a flight simmer.
If you are running a genuine ie Laminar Research Disc version then you need to have disc No 1 loaded into the disc drive for the problem to go away.
If you are running a digital download version then I am not able to help
If you are trying to run a steam version then you need to raise your query in the Steam forum. This forum is for the dedicated genuine Laminar Research product.