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Cloudflare is running an HTTP proxy server (i.e. kind of a web-broker) that has two sides to be managed. It is accepting web request from your user's web browser (e.g. Chrome, Safari) therefore the SSL certificates used by CloudFlare must be compatible and acceptable to your browser. Many times people choose to use DNS-only, but the website is not served properly yet. Sometimes the problem is due to type of SSL certificate you are using. Acquire some knowledge related to SSL certificates verification method.