Comment 28 for bug 1797945

Note: This may be fixed in some Weeks, Months, or Years, so maybe you can try first:
 > Thunderbird > Add-Ons > Extensions > Search for: Lightning > Install.
If you use Ubuntu-Linux or a Derivative, Lightning is not included in Thunderbird 60+ and the Update will break your Calendar (incompatible, not compatible). xul-ext-lightning is not international. Search for Add-On leads to incompatible Version. Someone told, the Beta is not recommended.

 > Download latest Version of Thunderbird fitting for you from here:
 > Open File with Archive-Program(Unzipper)
 > Goto > thunderbird > distribution > extensions >
 > {e2fda1a4-762b-4020-b5ad-a41df1933103}.xpi (=Lightning) > Extract
 > Open your installed Thunderbird > Add-Ons > Extensions
 > (Remove unwanted english Version of Lightning)
 > Gear-Wheel-Button next to the Search-Box
 > Install Add-on From File
 > Select Download-Folder and the .xpi-File > Install
 => All your Calendar-Data should be back