I recently am getting the following messages on the majority of my sites:
Sitemap is HTML
Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
I haven't added anything to these sites like plugins or made any changes actually since the problem started. Any ideas what could have caused it? Also all my post categories have disappeared, not sure if it's related.
There was server maintenance done but hosts insists all is good on their side.
WordPress SEO Version 18.104.22.168