I removed an SSL server certificate or disabled ACM for the custom domain. The app no longer accepts requests even at the default domain name,
$ curl -I https://<app-name>.herokuapp.com/ curl: (7) Failed to connect to <app-name>.herokuapp.com port 443: Connection refused
Please enable Automated Certificate Management:
This will trigger the platform to provision the SSL server certificate for the default domain name. In addition, if your DNS is configured properly, the SSL server certificate will also be valid for the custom domain.