Geofield (geocode from another field) - database entry not created
So in the end the problem actually had two causes, which made it more difficult to solve than it should have been:


a bug in the geocoder codebase (solution: apply this patch)
the Google Maps API key used in the geocoder configuration was actually invalid for this type of operation, and so I had to generate another key specifically for geocoding operations. (I noticed the problem via a geocoder error saved in the recent log entries pages.)