(Resolved) Error 521 CloudFlare

Whenever we turn on proxied mode we get a web server down (error 512) error. I have googled a lot in this forum and we tried multiple things without luck:

  1. Web server is responding on HTTPS (443) without CloudFlare
  2. Universal SSL has been turned off then turned back on
  3. Purge All Cache has been used many times
  4. Tried all SSL/TLS encryption mode available

It seems opening support ticket is not possible anymore. My domain is “onecdn.org”. Could somebody have a look and advise?

Thank you

SOLUTION: I switched SSL from flexible to full (strict) and then solved the problem though I tried it previously and didn’t work so weird.