Hello,
Error 521 occurs when the origin web server refuses connections from Cloudflare.
We wouldn’t be able to check further as Cloudflare is paused for the domain. Could you kindly unpause it ?
The two most common causes of 521 errors are:
- Offlined origin web server application
- Blocked Cloudflare requests
Kindly review this document for the resolution : Troubleshooting Cloudflare 5XX errors | Cloudflare Support docs
Thank you !