In “Edit Page” editor, the Sections box (“Add Section”) is totally missing. (“Use Sections” is turned on. an “Screen Options” Sections is checked). There is an empty space for the Sections box. Now I can’t edit any of my sections anymore. I already tried to disable all other plugins but that didn’t help.
Thanks for your quick response. That’s really weird because I haven’t had time yesterday and suddenly today everything works fine. I just found out that in the night there has been an automatic update to WP 4.9.1. So maybe that resolved the problem…