Error with Google XML Sitemaps

Using your plugin “Warm Cache” together with the XML sitemap generated by the “Google XML Sitemaps” plugin I always get the following error, no matter what configuration changes I try.

A notice from plugin Warm-cache: Your configured sitemap url ( https://dusi.ro/sitemap.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.
Error detail: WP_Error Object ( [errors] => Array ( [http_request_failed] => Array ( [0] => cURL error 28: connect() timed out! ) ) [error_data] => Array ( ) )

PS – sitemap URL is: https://dusi.ro/sitemap.xml

Thank you for your help.