Well, I found the solution: If You choose a document layout that changes the font family in sections LyX does not allow you to overwrite it in the document.
Seems like that's just the beginning of a long journey for me
today I tried out LyX and I have to say that I am a little frustrated. But I hope it's because I don't know enough yet and You can bring light in the dark ;)
I wrote a little text with a title, some sections and subsections. When I checked the pdf-output I found, that the whole ...