There have been recent reports that Cloudflare was blocking only the skin style load.php request and returning a HTTP error 520 for it. For instance urls like:
load.php?lang=en&modules=ext.visualEditor.desktopArticleTarget.noscript%7Cskins.timeless&only=styles&skin=timeless
This was because in MediaWiki 1.38, the installer allows you to upload a logo and this logo gets set as a data uri. This datauri can be VERY large. This data uri is also propagated via the Link: http response header on the mentioned skin module and can cause the headers to become so large that Cloudflare will not allow them and will return this error 520. See also phab:T318752 and phab:T318753.