ERR_NGROK_628: The API key is already owned by a user and cannot be re-assigned ownership - ngrok documentation

What does ERR_NGROK_628 mean?

ngrok returns ERR_NGROK_628 with this message:

The API key is already owned by a user and cannot be re-assigned ownership.

Where can I get more help with ERR_NGROK_628?

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