icloud calendar won't sync with Thunderbird/Lightning

[quickquestion]

I've used Lightning successfully for more than a year. Recently, sync stopped working. A pop-up windows asks for the password, again and again, but will not "connect".

I am not using two-factor authentication with icloud, and prefer it that way.

Is anyone currently syncing Thunderbird with their icloud calendar via the Lightning add-on and NOT using 2-factor authentication?

[marcelstoer]

I've recently gone through the same ordeal and have therefore documented the whole process: https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/

Due to 2-factor authentication you now need to generate an app-specific password in your Apple ID account.