Constant warm cache errors, no response from author

Keep getting the error:

A 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.

Even though site map is configured, resolution was to remove warm cache simply because author would not respond to the issue.