521 Error

This 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:

• Offlined 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
• Whitelist all Cloudflare IP ranges in your origin web server's firewall or other security software

Source: https://support.cloudflare.com/hc/en-us/articles/115003011431#521error