If you're looking for information regarding Cloudflare
server error 501, you've come to the right place. Cloudflare error 521 indicates that TCP connectivity to your source server is not permitted. Cloudflare attempted to connect to the main server through either port 80 or 443, but received a connection rejected error. When the origin server explicitly denies Cloudflare's proxy request, security or firewall software often generates Error 521.