Main Menu not visible on some pages

So here is a weird problem - my main menu suddenly disappears on some pages...

See here: - and basically nearly every other page, there is the main menu.
However, watch this page: no menu, nor a language selector... however, if you downsize your window on the pc that much that the mobile menu applies that appears...

I used to have the same problem here until I changed the site template to use from "blog" to "blog masonry" - however, it does not change anything on any post or in the event calendar.

Any ideas on how to find out the problem here?

(If someone answers I can grant support access. However, as the site is secured twice I would have to know first, who's gonna take a look and then send a password to even access the support page :wink: )