xml opening tag issue

Hi, i’ve switched to https and since then have started getting this message.

A notice from plugin Warm-cache: Your configured sitemap url ( https://www.######.co.uk/sitemap_index.xml ) is configured, but does not appear to contain an xml opening tag, or a combination of urlset or sitemap, I cannot crawl your pages. Please check your sitemap plugin to fix your currupt sitemap.

Note: this check will be cached for 2 minutes. So if you fix the problem, this notice might still be present for 2 minutes.

Been using yoast sitemaps and have been successfully using warm-cache plugin for a couple of months. Hoping a fix can be found.

PS. I can’t believe this isn’t a more widely used plugin, genuine cache pre-loading seems vital for SEO, and preloaders in WC and Total just aren’t comprehensive enough. With google using averages, the entire site needs cached at all times to keep that average up. Thank you for a great tool, and sorry I seem to have broken it.