Google shut down version 1 of their calendar API and the google calendar provider suddenly stopped working. See [1] for an explanation from the gdata-provider developers. This can be fixed by updating gdata-provider to version 1.0.2, which uses v3 of the Google Calendar API. [1] https://addons.mozilla.org/en/thunderbird/addon/provider-for-google-calendar/ Reproducible: Always
The way gdata-provider (and lightning) is integrated into thunderbird right now is not a particularly great fit; I will have to look into this further to figure out the best method to get a new gdata-provider in place.
Isn't this bug resolved? Using mail-client/thunderbird-31.3.0 I can sync my Google calendars again. Extension is displayed as '1.0.3pre2' in addon manager.