I've had the "1 authentication pop-up per subscribed calendar"-problem since the add-on version 1.0 and I've tried everything so far listed in this post. After I've removed all the calendars and try to add a new one, I go the steps from "On the network" -> "Google calendar" to a screen that says "Locate your calendar".
It has one entry field, which is empty and a radio button next to it. If I hilight the field, the "Next" button gets grayed out. If I press the Next-button, I usually get an empty list of calendars.
If tried again, I at times get the Google login screen, after which the calendars are listed, but then if I choose any of them, I get as many new login pop-ups as the calendars I checked. After restarting TB the same amount of login pop-ups appear again.