Hello,
Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain Cloudflare IP addresses.
As a resolution, Contact your site administrator or hosting provider to eliminate these common causes:
- Ensure your origin web server is responsive
- Review origin web server error logs to identify web server application crashes or outages.
- Confirm Cloudflare IP addresses are not blocked or rate limited
- Allow all Cloudflare IP ranges in your origin web server’s firewall or other security software
- Confirm that — if you have your SSL/TLS mode set to Full or Full (Strict) — you have installed a Cloudflare Origin Certificate
For further information, kindly review this document : Error 521: web server is down
Thank you