Error 521: Web server is down when origin server is google cloud

[nob]

Recently I do migration from cpanel to google cloud. Then I set my external IP in Cloudflare’s DNS,

• Without Cloudflare proxy, its running good,

• But when I use Cloudflare proxy, it shows Error 521: Web Server is down.
  For overcoming this, I update ufw, as follow: GitHub - Paul-Reed/cloudflare-ufw: Script to update UFW with Cloudflare IPs, this script allow Cloudflare IP ranges whitelisted.
  But the sad thing is, still I get Error 521: Web Server is down.
  I searched over the google, but still I can’t solve the problem.
  What should I do now?
  My site: https://newsofbangla.com

[sdayman]

Before you added it to Cloudflare, it was working with HTTPS?

[NicolasGo]

I tried 1 minute ago and it was working.
Just now it seems you’ve got a certificate problem and it’s down (prefere Full encrytion [Encrypts end-to-end, using a self signed certificate on the server] to full strict.

[nob]

No Bro

[nob]

You saw the lets encrypt ssl. But the problem still not fixed.

[nob]

I observe that, when i enable universal ssl then my https website shows 521 error and its still not solved.