Hi @mcheah,
Welcome to the Auth0 Community!
This was a known issue from our end and we are sorry for the inconveniences that it has created. The underlying problem has already been addressed and resolved by our engineering team and the system is now operating normally.
Please let us know if the issue still resides, since otherwise the recommended workaround would be deleting and recreating the custom domain. Thanks!
Best regards,
Remus