Hi there,
So the Site is using GP Header Element:
https://docs.generatepress.com/article/header-element-overview/
The Display Rules for the current Header is set to the Entire Site which includes the blog page. Add Blog to the Display Rules > Exclude.
Now create a new Header Element with the exact same settings as the current one.
Change the Content of the Header Element from <h1>{{post_title}}</h1>
to <h1>Aktuelles</h1>
and add Blog the Display Rules > Location. No exclude rules. Or anything else.