Error 521 - aws web server refuses connections from Cloudflare

I have a website and I show frequently error 521.
I have checked that:

  1. Web Application Firewall (ModSecurity) is disabled for the domain.
  2. Cloudflare IP addresses were added to the nginx configuration and nginx was restarted.
  3. Cloudflare Ip addresses are not blocked by Fail2ban (Fail2ban was temporarily disabled but the issue was not resolved, so it was enabled again).

Using AWS LightSail Cloud service.
Plesk Web Hosting, recommended to contact Cloudflare support directly to address this issue as Error 521 is a Cloudflare-related error and they may tell exactly why it appears.

Yes. It’s work.

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