Hello,
521 error referring to webserver down.
You may want to check if the origin servers block Cloudflare IPs or Port 443 and Port 80.
Cloudflare IP range
I would strongly suggest you to check with the origin server provider.
I’ve run test to the CNAME target IP, and it’s showing port 443 (tcp) failed: Connection refused and Failed to connect to x.x.x.x port 443 after 220 ms: Couldn't connect to server.
Both Port 443 and Port 80 showing connection refused.
Redacting the IP for security purpose.