This means that the error is not generated on the server (Plone) side.
My first assumption has been that the error comes from Google -- as a special means to indicate problems with the access to https://maps.googleapis.com/maps/api/js?key=BLAHBLAH&callback=initMap. The HowTo you reference in your initial post does not contain the callback=initMap. I would carefully check that you really follow the HowTo.