the plugin injects wp-content/uploads/rc_geositemap.xml which doesn’t exit (returns 404). the author committed to fixing this issue more than a year ago, but it’s still there. this non-existing sitemap appears in sitemap_index.xml (created by yoast SEO, but the problem comes from this plugin, not yoast’s one). google webmaster tools reports this as a broken xml.

https://wordpress.org/plugins/rich-contact-widget/