Different page header image for a page

https://www.nextgamers.eu/game-servers/minecraft-server/ that one with a green background.

Just to clarify, you added a page header and merged it with the site header? Did you add any content to the Page header?

no i didnt add any content to the site header.

The page header needs something in it for it to work. So just add an empty <span></span> tag. And then adjust the top and bottom padding to suit.

Where do I add that <span></span> tag? oO I've tried writing it into the editor-field in elements at page header, and i tried creating a new page header with content <span></span> but both didnt work.

The editor field is the correct place.
Can you provide a screenshot of the page header settings you are using?

Here a screenshot of page header section: https://imgur.com/a/q9HSHBy

Aah ok, now i see, so the Page Hero is where you can add a background image (or let it use the pages featured image). Which needs to have some content for the image to display. You can add padding to that to increase the size if needed.

Then remove the color from the Site Header so it doesn't block the image and adjust the Offset Site Header Height so your content doesn't fall behind it.