Created attachment 673977 [details] [diff] [review] fix In Lightning 1.9 the API for cached calendars was changed in such a way that it breaks compatibility with previous releases. Updated calendar providers will break if used with an older version like Lightning 1.8. Provider for Google Calendar extension claims to be compatible with Lightning 1.1 and Thunderbird 8. This is wrong. It requires at least Lightning 1.9 and therefore Thunderbird 17 to work correctly. The attached patch raised the em:minVersion to the actually supported versions.