Thanks for help. I could fix the problem by deactivating the plugin. It was WordPress Popular Post Plugin V6.0.0 and by looking into the plugin thread, it seems I’m not the only one having trouble after the last plugin update.
Hi,
I made a screenshot how it looks like here.
I can make new posts or edit old ones with Elementor. WP Quick Edit does also work. Normal WP Editor ends up in the attached screen.