Dr. Adams at http://psychological.com had a problem getting rid of google sitemap errors. After resolving the problem he sent us this update:
If you use Youst as an SEO plug-in for WordPress, do NOT use Youst for creating a sitemap. If you fail to uncheck a box, it creates a sitemap.HTML, and Google becomes obsessed with it. Instead use the Google plug-in for Sitemap.
OK, so you now have Google believing you have sitemap.html…even though such a file does not exist.
So you see a list of errors in Google Webmaster Tools, and you will never get rid of it. Never, ever…for eternity. Trust me.
It occurred to me that every stiemap.xml (sitemap-authors.xml) that is now misidentified as .html.
So you take each of those misperceived *.xml files and test them in Webmaster Tools. They will return NO errors. You then submit them one at a time. Each has many sitemap files nested beneath.
So each one you test/verify has many files beneath it.
Then you submit each (which includes nested files) again. This time they will pass.
I had 66 sitemap errors, all listed as html even though they are not.
Once you verify and then submit again, they go away.