Sitemap could not be read

i have another Wordpress website (already several "customer websites") where i take care of the indexing, but with this installation i get the errors "Sitemap could not be retrieved" if i click on the message to display more it says "Sitemap could not be read", unfortunately without further information. I waited for a while because sometimes google takes a while(4days), unfortunately without success, but the sitemaps are easily accessible via direct link, since we are using the same seo plugin as always (yoast seo).

  • reinstall plugin
  • check the xml file (it is readable etc)
  • deleted the sitemap after 4 days of waiting from GSC and added it again

https://cleverdigital.de/sitemap_index.xml