Ok. I have no idea what causes this problem.
But instead of going on with knocking my head onto the table I decided to keep on designing the rest of the website.
The “section 2” which causes the bug, had a height of 100vh, when the bug occurred.
I changed the height to 80vh because that’s fitted the design better and now the interaction in the header section works again.
I have no idea why and what causes the problem in the first place, but hey, it’s working