I had no html file, so I renamed the sitemap.xml to sitemap.html but that did nothing to correct the problem. I noticed when changing it to html it becomes a Firefox html document, but opens with Explorer. Could that be the problem?
My original files were sitemap.xml, sitemap.xml.gz, sitemap_news, sitemap_video, sitemap_images (which all 3 are xml files) and urllist.txt.gz file. Did not have any sitemap.html file originally.