Hi Dietmar (@dgiljohann)!
I can somehow not reproduce the error although I had it yesterday. Could you try it again and post a screenshot of it? I would say that you need to have solids in your geometry. I am not quite sure if Salome is able to close shells of your geometry properly. But that is something I would take care of.
Please let me know if I can help you with anything else and do not forget to contact me if any errors occur.
All the best!