With one of the latest updates of Thunderbird the Cardbook add-on stopped working for me. I’m using Ubuntu 20. After a little browsing I found the solution.

Manually update the cardbook plugin. I downloaded the add-on from the Thunderbird Add-on Directory and installed it through the add-ons screen in Thunderbird.

It seems the plugin doesn’t automatically update and the old version isn’t compatible with Thunderbirds latest releases. See the Cardbook issues-page on Gitlab for more details.