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Hi Zeno: I can not find any reference to the calendar in Add-Ons I thought the calendar is integrated in Thunderbird. Or do I have to add Lighting as an add-on? Thanks!

Lightning is the calendar.

Calendar was working until update to 45.2.0

so I need to reinstall the Lightning add-on?

Lightning is an add on. If it is not displayed on the add on page it is not installed. So yes you need to install it.

Thanks!