ERR_NGROK_20004: Failed to establish a connection between the Upstream and Target Services - ngrok documentation

What does ERR_NGROK_20004 mean?

ngrok returns ERR_NGROK_20004 with this message:

Failed to establish a connection between the Upstream and Target Services.

Where can I get more help with ERR_NGROK_20004?

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