Now I understand: With the latest mobile view versions, navigation menus are supposed to appear after the content.
Yes, that's correct: My customer prefers the menu to appear on top and he has fair arguments. That lead me to the idea to put another navigation module into the header, hiding it by CSS on the desktop view and hiding the initial navigation menu (sidebar) on the mobile view. Well then, the menu appears on top of the content and another customer is happy again.
Thanks for your help, Egor, and have a good weekend.