We're running a lot of sites and we've started to get a lot of these errors in Webmaster Tools:
Sitemap is HTML
Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
One of the problematic sitemaps:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://www.same_domain.co.uk/folder/file1.shtml</loc>
<lastmod>2011-05-11</lastmod>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>http://www.same_domain.co.uk/folder/file2.shtml</loc>
<lastmod>2011-05-11</lastmod>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>http://www.same_domain.co.uk/folder/file3.shtml</loc>
<lastmod>2011-05-11</lastmod>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
<url>
<loc>http://www.same_domain.co.uk/folder/file4.shtml</loc>
<lastmod>2011-05-11</lastmod>
<changefreq>weekly</changefreq>
<priority>0.5</priority>
</url>
</urlset>
Why would GWTs think this is anything but XML?
(Server: IIS)
Edit:
"This document was successfully checked as well-formed XML!" -W3C Validator.
Edit:
I resubmitted two problematic sitemaps, one with no changes, and one with a couple of extra lines to ensure it's treated as XML. Ran the "Fetch as Googlebot" diagnostic tool. Both are fine now. I'm just going to re-submit all sitemaps with the "Sitemap is HTML" error
The question remains: