Google is moving away from the APIs we're using, as described in this timetable:
https://developers.google.com/+/api/auth-migration#timetable
Although the timetable says the API is good until Sept 1, 2014, we're seeing the API fail on secure.phabricator.com and have received user reports of failures as well.
I can't figure out any configuration changes that users can make to keep the current APIs working, so just upgrade to the new ones.