[Plugin: WordPress SEO] ERROR: sitemap is not a valid feed template.

On that form you can add exceptions, if you add this one:

sitemap(_index)?\.xml

it should work.

Yup, now I got a normal 404 instead of the invalid feed error: https://jelmerdejong.com/sitemap_index.xml

Hmm that’s freaking weird. Could you save settings on the XML sitemaps page one more time?

Ah wait and now WP isn’t serving the error, you should either disable that feature OR add the exception mentioned above.

Exception added, now the feed error is back. Resave of sitemap did not help. https://jelmerdejong.com/sitemap_index.xml

Any chance of giving me admin access so i can check it out?

what is your email address?

joost at my domain.

Turns out my problems was caused by the theme I’m using, that has a feed rewrite rule active. The latest post of @yoast helped me to fix this: https://yoast.com/xml-sitemap-in-the-wordpress-seo-plugin

I removed the redirect code from the theme file and disabled the rss rewrite in the themes admin menu.

Cool.