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Error 521 typically arises when Cloudflare requests are not being responded to by the web server. When someone tries to access your site, Cloudflare acts as an intermediary between the visitor and the server where your site is hosted. If the server is not responding properly, Cloudflare displays error 521 on the website.

Common reasons for Error 521

-Origin web server is offline. Its possible that the server is either offline or there is an issue with Apache or Nginx config.

-Configuration issues with the origin web server. One possible cause of error 521 is a misconfigured server. Servers needs to be specifically configured to work with a CDN service.

-Issue with the SSL certificate. Its possible that misconfigured SSL on the webapp can cause website not responding to https requests.

How to find the root cause and resolve the Error 521

1) Check the origin server. In order to troubleshoot Cloudflare’s error 521, make sure that the origin server is online by checking the nginx/apache status and ensure your server is up and running. 

2) Check if the Webserver ports are open. Make sure that the ports 80 and 443 are opened in the server firewall as well as in your server provider’s external firewall. You can verify the ports at https://portchecker.co/checking 

3) Install SSL certificate. Usually, installing an SSL on the webapp will open the port 443. In order to install SSL, go to the Web Application  > SSL > Select the SSL type and click on Deploy the SSL.

4) Configure SSL properly. If the SSL is not installed properly or the SSL config causes any issue with nginx, webserver wont respond to https request. You can try removing the SSL from the webapp, restart nginx and re-install the SSL.

5) Check your webapp error logs to identify any specific issues that may be causing the error. You can check the access log by going to the Webapp > Webserver Logs > Nginx Error Logs

6) Configure the firewall properly. Check your server’s firewall settings to make sure it is not blocking Cloudflare IP addresses.