Thunderbird - Calendar (lightning) disabled

I just completed an update from 42.1 to 42.2 using ‘zypper dup’ method.

Thunderbird 45.7.0 is installed from the 42.2 update repo.
It opens OK, connecting to several email accounts in my .thunderbird profile from 42.1.

However, my Calendar tab has disappeared.
When I look in Tools-Add-ons Manager, Lightning is grayed out (disabled) and the only option button is ‘Remove’.

I realize that the Lightning Add-on is integrated into Thunderbird since TB ver38, but it seems it is still managed as if it was a downloaded add-on.

Any idea how to re-enable?

Found it!

Browsing the Thunderbird Error console (…Tools-ErrorConsole) I saw an error related to another Calendar related Add-On, LightningButton.

I disabled LightningButton, restarted Thunderbird and Lightning started up.