For whatever reason, the menus are missing on some pages. I can’t seem to find any difference between pages, that show menus and pages that don’t. It has nothing to do with user’s rights. And there are no local menu appearance configured for individual pages. In the chrome developer console’s network tab, there are no http errors (everything is 200).
example with menu: [BPE 3.2 Funktionsgraph]
example without menu: [BPE 3.4 Polynomgleichungen]
did you notice the menu hiding all the way at the bottom left on the ‘example without menu’ ?
Are you using any specific css or html on the non menu pages?
thanks for your immediate response!
no, I actually didn’t notice … thanks for the hint! it’s not visible in my chrome browser, but I can find it in the page source. Now that I know, that it is actually rendered, I found out, that the dom structure of these pages is somewhat different:
this is the structure of a page, where the leftPanels are visible:
Is it possible, that some pages were not migrated properly during an xwiki update or maybe some pages existed prior to this update and others not?
(btw: my custom css is global an applies to both pages)
this is the structure of a page, where the leftPanels are invisible:
I dare not say how you got to that situation, this is how it looks on our xwiki;
with P being the panel and main being the page stuff.
I solved it 
there was an errorneous macro on the page with the following code:
...
<div>
|**Bearbeitungszeit gesamt:**|$mins min
</div>
instead of
...
<div>
|**Bearbeitungszeit gesamt:**|$mins min
</div>
the closing div got into the table cell and so it was missing behind the table. The effect was, that the content had one closing div missing
Again, thanks for your help!