ERR_NGROK_1668: Invalid endpoint configuration, email domains must be lowercase but DOMAIN is not - ngrok documentation

What does ERR_NGROK_1668 mean?

ngrok returns ERR_NGROK_1668 with this message:

Invalid endpoint configuration, email domains must be lowercase but <DOMAIN> is not.

Where can I get more help with ERR_NGROK_1668?

Email support@ngrok.com and include the error code.