Regarding your server, we see Cloudflare has the following configuration modes: Off, Flexible, Full and Full Strict (see https://www.siteground.com/tutorials/cloudflare/ssl/). We understand you chose Full Strict, if not please let us know.
Next, your HTTPS seems to be successfully configured. Can you access your domain on HTTPS and port 443, and see the certificates are configured as Cloudflare Origin?
In order to do this, you could access the server (the one using Cloudflare) by its IP and see if it works.