When you open this page, scroll down until the hero section is out of view, then reload the page and scroll up to the top, the background image sits about 20 pixels below the top.
It does not matter if the image size is set to cover, contain or custom, scroll or fixed or what the position is (top, center left, asf.)
Animation:
https://eup.ke-server.de/images/sampledata/etter_scroll_error.gif
Screenshots:
before
https://eup.ke-server.de/images/sampledata/bild1.png
after
https://eup.ke-server.de/images/sampledata/bild2.jpg
(This happens in Chrome, Edge and Firefox btw)