domain.com/sitemap_index.xml give 404

I updated YOAST SEO to Versión 5.4.6 and goes to Sitemap to change somethig wrong, save configuration and…. sitemap_index.xml page not found 404.

Google Webmaster tools give me the same error.

Any help? I have to webs and this 404 in both!

Thanks

https://wordpress.org/plugins/wordpress-seo/