I am using a custom Domain Name with my fly.io app. For some reason, the X.509 Certificate didn’t get auto-renew. This leads to the inaccessibility to the fly.io app. I suspect it is because by the Cloudflare DNS Proxy which causes the custom domain verification to fail. I had to stop the proxy, run fly certs delete <custom_domain_name> and fly certs create <custom_domain_name> to get a valid X.509 certificate again