"message": "Invalid redirect_uri: Given URL is not permitted by the application configuration.",
Your redirect url: http://dennys-m.appspot.com/fb_login/ must match the `canvas url` in https://developers.facebook.com/apps/ configuration panel.
Fortunately, these days Facebook gives the error message `Invalid redirect_uri: Given URL is not allowed by the Application configuration`. About a year ago no such exception appeared, just strange results.
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