Since I could not identify the error I finally copy/pasted the example code from Google, which can be found here: https://developers.google.com/+/web/people/#retrieve_an_authenticated_users_email_address
The only thing I changed was the Client ID (for web applications).
And I STILL get the same error!
Since the example code does not work for me, I'm thinking, maybe my settings in the Google API Console are wrong. I have tried to follow the instructions:
You must include the script and include the
client API in addition to the
The demo page was doing it incorrectly, which I've fixed.