Websites hosted on a Plesk server that use Cloudflare display Error code 521

Symptoms

• Websites that are hosted on Plesk servers and use Cloudflare are opening intermittently, while sometimes showing a Web server is down Error code 521 page
  
• When manipulating the local hosts file and loading the website directly via the server IP (in order to bypass Cloudflare) by using the instructions in this article, the websites are always loading fine
• There are no errors within the web server logs for any of the affected domains

Cause

The Plesk web server is loading fine, however the Cloudflare DNS service is not able to reach your Plesk server due to current issues Cloudflare themselves are experiencing:

Resolution

The only available ways in which such an issue can be resolved are the following:

Option 1
Stop using Cloudflare DNS services for all of the affected domains and switch the nameservers domains use to local nameservers instead of Cloudflare nameservers.

Option 2
Wait for Cloudflare to resolve their own DNS issues while keeping an eye on their https://www.cloudflarestatus.com/ page.