I just recreated the same exact setup that worked in a dev tenant in a production tenant with a custom domain. Everything goes smoothly except the actual loading of the SSO dashboard. The logs show it successfully logs into the dashboard but when it gets to this step of the login process:
https://<CUSTOM_DOMAIN>.us.auth0.com.us.webtask.run/auth0-sso-dashboard/login?code=
There is no response from Auth0, there is an error in the browser concerning the SSL connection.
<CUSTOM_DOMAIN>.us.auth0.com.us.webtask.run uses an unsupported protocol.
ERR_SSL_VERSION_OR_CIPHER_MISMATCH