Rated 5 out of 5 stars

I had the same problem with login to Google as https://addons.thunderbird.net/sk/thunderbird/addon/open-google-calendar/reviews/1163926/
The page said: "The browser you're using doesn't support JavaScript, or has JavaScript turned off."

The solution was quite simple. Instead of clicking the existing account on "Choose an account" page, I clicked "Add account" link. This allowed me to enter my google credentials again and everything worked fine.

This review is for a previous version of the add-on (1.5).