prevSibling and nextSibling not working in blog with moved posts

Starting with grav-skeleton-blog-site I moved the blog from the home page to another page to have it visible only inside a member area. Homepage is similar to the one-page skeleton.

This works, but the previous and next buttons from blog-item.html.twig always point to the blog item itself, not to the siblings. A pageing inside the post is not possible. What did I make wrong, how can I fix this? What is the purpose of the blog_url: /mitglieder setting in and in

My configuration:

ls pages/05.mitglieder/
01.personalverwaltung  glass-2006766_1920.jpg  posts

blog_url: /mitglieder
            '@page.children': /mitglieder/posts
    limit: 6
        by: date
        dir: desc
    pagination: true
    url_taxonomy_filters: true