UPDATE Friday, September 28, 2012
Provider for Google Calendar extension version 0.16 now works with Lightning 1.7 with Thunderbird 15.0.1. If you’re having trouble, I recommend you uninstall the extensions you have, restart Thunderbird, and reinstall them.
ORIGINAL August 28, 2012 post:
Mozilla pushed Thunderbird 15 to me today, which I installed along with Lightning 1.7.
Some very cool new features (like integrated chat and slick new UI)
BUT – Tbrd 15 broke my Google calendar integration. I could no longer create events and could not dismiss events already in my calendar. Essentially, all the write functions to my Google calendars were broken.
THERE IS A FIX!
Thunderbird 15, Lightning 1.7 – Google calender broken UNTIL I stumbled upon the 0.16pre version of provider here:
Using the direct link to the gdata-provider.xpi add in is tricky using Firefox because Firefox thinks it’s supposed to install the plug in, downloads it, recognizes the plug isn’t Firefox, then quits.
To make this work using Firefox go to http://ftp.mozilla.org/pub/mozilla.org/calendar/lightning/candidates/1.7b3-candidates/build1/unsigned/win32/ and right click or CTRL click as appropriate the gdata-provider.xpi file choosing the “Save as…” option. Save the file to a convenient temporary location.
Then go to Thunderbird and open Tools>>Add-ons. In the upper right hand corner, click the gear. Choose “Install Add-on From File…” Find gdata-provider.xpi that you saved to disk and let Thunderbird install it.
Next, check that you have Lightning 1.7 installed. If not, use the Get Add-ons tab, search for Lightning, and install it.
That trick worked for me. Hope it works for you!