This XML file does not appear to have any style information associated with it.

It appears your main sitemap index returns an empty response. Are you seeing any related errors in your server/hosting logs?

Do you know when this started? It may be a conflict with another plugin. Or a conflicting .htaccess rule…

Alternatively, you could deactivate the XML Sitemap Index option on Settings > Reading and install/activate https://www.ads-software.com/plugins/xml-sitemaps-manager/ which uses the WP core sitemaps but allows you to manage them similarly to this plugin.

The problem exist only at root / sitemap index
https://sonntagsnachrichten-herne.de/sitemap.xml
the other sides works
https://sonntagsnachrichten-herne.de/sitemap-news.xml
https://sonntagsnachrichten-herne.de/sitemap-posttype-post.2021.xml
https://sonntagsnachrichten-herne.de/sitemap-posttype-post.2022.xml

Chance to “xml-sitemaps-manager” is no solution.
.htaccess is okay there are no changes.
No conflict with other plug-ins I think.

I payed for support and XML Sitemap Advanced but, I cannot change the name of the sitemap index also. ???