Comment 60 for bug 1850651

"Google Provider" is a 3rd-party add-on that hasn't been updated to work with the new Google OAuth2 scheme. Complain to its author:
https://addons.thunderbird.net/en-GB/thunderbird/addon/provider-for-google-calendar/
https://github.com/kewisch/gdata-provider/

In certain upgrade situations it can happen that TB doesn't recognise your existing profile and creates a new one. You can select the profile want in the profile manager or via "Help > Troubleshooting Information", about:profiles. We're working on improving that.

To my knowledge, Thunderbird's Calender Lightning will work in any version you install.

For security reasons we must eventually discontinue old versions and migrate users to the new supported version. The update from 60 to 68 has been a bit bumpy for some users which we regret.