I could not receive callback on other than 8080 port.

Maybe the reason is that redirect uri port number is not reflected to CherryPy server settings.

I added CherryPy server hostname and port settings same with redirect uri values.

Please check with another port number setting like this.

# gather_keys_oauth2.py

redirect_uri = 'http://127.0.0.1:8081/'
server = OAuth2Server(*sys.argv[1:], redirect_uri=redirect_uri)
server.browser_authorize()