Comment 44 for bug 545778

You can find Workarounds here:
(https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1797945)

If you want the english Version of Lightning, type into a Terminal:
sudo apt update
sudo apt install xul-ext-lightning

If you want a localized Version of Lightning, at the Moment, you cannot use the normal Add-On-Search in Thunderbird (incompatible).
Download manually the latest Beta-Version of Lightining for your System- and Language-Version from:
https://archive.mozilla.org/pub/calendar/lightning/candidates/

Open Thunderbird > Add-Ons > Extensions > (Remove unwanted english Version of Lightining) >
  > At the top of the page, click the Gear-Wheel-Button next to the Search-Box to open a menu
  > Install Add-on From File
  > Select Download-Folder and the .xpi-File > Install
=> All your Calendar-Data should be back