Google Calendar integration into Thunderbird suddenly did not work any more for me. Thunderbird kept asking me for my password to access calendars, but did not show them when given the correct credentials. Instead, calendars in the list got the famous "yellow triangle".

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I got it working again doing the following.

1. Remove the Ubuntu packages the "Lightning" and "Google Calendar Provider" Thunderbird addons:
   ^
   sudo apt-get remove xul-ext-gdata-provider xul-ext-lightning

2. Restart Thunderbird

3. Install the addons using Thunderbird's built-in addon search:
   ^

   • Open Menu "Tools" → "Addons" → "Get Add-ons"
   • Search for "Lightning", and install
   • Search for "Google Calendar Provider", and install

4. Restart Thunderbird

Your calendars from before should appear again automatically. \
Lightning might ask you again for your password, but it should work now.

Maybe swapping only one of them out for the non-Ubuntu version (I'd assume the Google provider addon) would be enough, but I really didn't care.

If you see "No writable calendars configured for invitations" for incoming invitations, you might want to choose this setup.