Ok, it was a conflict with the NextGEN Gallery. I deactivated the Plugin, now it works. Strange. But what if i need a gallery?
Another question: The feature “Sections” doesn’t work with Gutenberg. Is it possible to activate the Classic Editor only sitewise, not for every page/article? Otherwise i would lost some Gutenberg features i implemented at some places as by now… or do i have to decide? What happens to features i already had implemented via Gutenberg if i switch?