A 521 error happens when we are unable to make a TCP connection to your origin server, typically because the connection was refused which often can be caused by security or firewall software.
Because Cloudflare operates as a reverse proxy the IP address your server will see is one of a limited number of Cloudflare IPs. In that sense, many actual visitors may all come from the same IP address, which can cause firewalls or security software that is not appropriately
allowlisting the Cloudflare IP ranges to block this traffic as it may see it as excessive or malicious. We publish a full list of our IP ranges here so that you can
I would make sure that your hosting provider confirm that the Cloudflare IP ranges listed in the URL above are fully allowlisted from any security software, firewall etc to ensure there is no rate limiting or blocking of our edge servers. This should ensure that Cloudflare can consistently make a connection to your origin server to retrieve content and serve it to your visitors