Sitemap Error: Unsupported format HTML
Unsupported format. Your file appears to be an HTML page.
Was the correct sitemap URL submitted?
The Yoast SEO sitemap URL to submit ends in sitemap_index.xml. If you submitted sitemap.xml, please remove it and submit the correct sitemap.
Does the sitemap error?
Open the sitemap in a browser. Remember, the Yoast SEO sitemap URL to submit ends in sitemap_index.xml. Does an error appear? If so, search our list of common sitemap errors for additional steps.
Sitemap redirects to another page
Redirects can come from WordPress (core, theme or plugins) and your server (htaccess or webhost control panel). A conflict check can help narrow down where the redirect is located so you can remove or modify the redirect.
Hidden Video Sitemap is cached
Typically this happens with video sitemaps when you’re running W3 Total Cache and you’ve ticked the setting to hide the sitemap from normal visitors. What W3TC does is cache the error which normal visitors get, if they want to view the video XML sitemap. This error is then also served to googlebot, because the logic to show the sitemap to googlebot won’t execute. The best way to fix this is to create an extra user agent group.
You can create this user agent group by going to the W3TC’s settings. Under Performance and then User Agent Groups there’s a button “Create a group”. Click this button and call your group “Google” or “Googlebot” and then in the user agents field you just enter “googlebot”. This will force W3 Total Cache to make a separate cache for Googlebot user-agents.
Another option is to prevent W3TC from caching video-sitemap.xml entirely by excluding the sitemaps, but since that’s slower, we actually prefer the user-agent group.