Hello, an 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 
.
The two most common causes of 521 errors are:
- Offline origin web server application
- Blocked Cloudflare requests
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
- Find additional troubleshooting information on the Cloudflare Community .