notice from plugin Warm-cache: Your configured sitemap url ( http://www.mydomainname.com/sitemap_index.xml ) is configured, but does not appear to contain an xml opening tag, or a combination of urlset or sitemap, I cannot crawl your pages. Please check your sitemap plugin to fix your currupt sitemap.
The sitemap is good. I am using WP 4.42 and the most recent warm cache revision. Do you have a solution?