Events+ - Unable to connect a Facebook app to import events

On 2 sites on the same server, any Facebook app created to enable importing FB events into Events+ simply does not work.
When trying to authenticate the app, we get the following error message which is simply not true:
"Can't load URL: The domain of this URL isn't included in the app's domains. To be able to load this URL, add all domains and sub-domains of your app to the App Domains field in your app settings."