When I use cloudflare DNS, there will 525 error and 520 error sometimes. There’s no error when I use other DNS. I don’t know why. I use nginx in my origin server.
Look to see if there is a firewall at the origin. If so, make sure Cloudflare IPs are whitelisted. When 52X errors are found, it’s typically a firewall blocking Cloudflare.
yes, there is a firewall at the origin. But when I stop the firewall, the error sometime show again. Not always show, in some pages some img show 525 error
Whitelisting the Cloudflare IPs is a good idea and might be the answer.