ERR_NGROK_105: The authtoken you specified does not look like a proper ngrok authtoken - ngrok documentation

What does ERR_NGROK_105 mean?

ngrok returns ERR_NGROK_105 with this message:

The authtoken you specified does not look like a proper ngrok authtoken.
Instructions to install your authtoken are on your ngrok dashboard:
https://dashboard.ngrok.com/get-started/your-authtoken

Where can I get more help with ERR_NGROK_105?

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