There is a version 91.0.4 of the Provider add-on that corrects this issue:

"Re-use same key as 91.0.2 to avoid rate limit exceeded errors"

https://addons.thunderbird.net/en-us/thunderbird/addon/provider-for-google-calendar/versions/

Perhaps not entirely fixed yet with 91.0.4: https://github.com/kewisch/gdata-provider/issues/373

Thank you so much. It was driving me crazy. I thought it had something to do with me leaving the country and returning. I am sure it will update automatically to the new version. I appreciate your response.

This has happen to all my accounts today On Thunderbird 91.5.1 (64-bit)

If you just need to sync events and not tasks, use the built-in CalDAV: File/New/Calendar, On the Network, enter the gmail account, calendars are autodetected, enter the account password in the OAuth window to allow TB access (cookies must be accepted in TB Preferences for OAuth to work).

Jan 25, 2022

I had this problem for 2 days. Today I was offered an update to 91.5.1 (32 bit) and when TB restarted the problem went away! Since DJSbuilding above had it start on 91.5.1 (64 bit) I'm not going to hold my breath that TB will continue to work. I do hope Google and Mozilla have worked out the problem, though.

I did not receive 91.5.1 yet. Hope I get it soon

I'm running 91.0.4 on 64bit and cannot connect to any of my Google calendars. TB version 91.5.1

Authorization Error Error 403: rate_limit_exceeded

This app has reached its sign-in rate limit for now.

WTH

It has been days so far that I can't use my calendars. I have Thunderbird 91.5.1 (64-bit) and can't enable my calendars with Google. So frustrating! See attached screenshot. ...mary

I have 91.0.4 loaded and still had the problem. I got around it by unsubscribing to Google calendar and then re linking to it. Works so far though this may pop up again.

Nor

I unsubscribed to Calendar, restarted, subscribed again. So far so good

Exactly the same. Unsubscribed Google Calendar and then resubscribed and it seems to be working fine now.