ERR_NGROK_5321: The RelayState must be configured with the redirect URL for IdP-initiated logins - ngrok documentation

What does ERR_NGROK_5321 mean?

ngrok returns ERR_NGROK_5321 with this message:

The RelayState must be configured with the redirect URL for IdP-initiated logins.

Where can I get more help with ERR_NGROK_5321?

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