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Gave up on this add-on.
Just follow the procedure that Greg supplied in his comments on jan 19th 2021 for the CALDAV method.
That works fine.
See:
https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/reviews/user:5686801

Ta opinia jest o wcześniejszej wersji dodatku (78.0.1).