Here is your first Forum » How To Fix Error 521?

While Error 521 is a fairly common error with wordpress and there are many ways to fix it but right noe I am goint to dicuss one of the most sure-sort methods of how to fix error 521
If your WordPress site's server is up and running but you still get Error 521 while trying to access it, the next step is to whitelist all of Cloudflare's IP ranges to ensure that your server isn't blocking them.
Make sure these IP addresses aren't being blocked in. htaccess, iptables, or your firewall. You should also double-check that your hosting provider isn't restricting or banning IP requests from Cloudflare's IP addresses. If you're not sure how to accomplish this, contact your host's customer service. These IP ranges should already be whitelisted at Kinsta.