On two Linux machines I upgraded from 78.x to 91.0, I had to delete the current Google email and calendar settings ('Unsubscribe') and then re-add them, re-giving the app permission to access the gmail/calendar accounts. Everything then worked as expected.

Not relevant here, but a third Linux machine did not pick up the current gmail and calendar account settings so I had to start from scratch.

I'm using win10. Don't remember what version of TB that I was running, but am now on 91 (actually 91.0.1). I had done what you suggested (deleting existing definition and reinstalling them), but it didn't help. Can you provide more details of what you did; maybe I missed something?

basically you will need to "reset" the calendar stuff for google is my feeling.

Due to enhancement in the way Thunderbird get google calendars (it now auto detects your shared calendars so all you do it sign in and select them.)

So unsubscribe from your existing Google calendars and then use the menu to add them again. The flow of things will be changes and you will be asked for your Google username and password, then offered a list of calendars to subscribe to.

I tried to re-establish the link to my Google calendar with no luck, so uninstalled TB and reinstalled it. After the reinstall my link to my Google calendar started working again but I've lost the link to my daughter's calendar and my Google contacts are no longer exported to TB.