BirdieSync is no longer compatible with the new version of Thunderbird/Lightning
If you upgraded your version of Thunderbird, it may happen that your current version of BirdieSync is no longer compatible with this new version, because of changes that would require BirdieSync to be adapted and tested.
Check with BirdieSync menu "Help/Check for update..." or in BirdieSync download section if a new version of BirdieSync is available supporting your new version of Thunderbird. You may then update to a more recent version of BirdieSync.
If no version is available yet, then be patient, a new version should be published very soon. It's generally a matter of a few days, sometimes less. Some adaptations and testing must be performed with the official release. Best is done to deliver you a new version quickly. But if you don't want to wait for the new release of BirdieSync, then you may choose to downgrade your version of Thunderbird.
Note also that if the date of this new version of BirdieSync falls after the 1 year period of free updates of your license, it may be necessary to upgrade your license. You may find more details in this FAQ entry.