Thunderbird needs to login google every time (CalDAV)
I also had this problem, and just fixed it. For me, the problem was that I recently changed my Google password, and I think Lightning was still using the old one. In the pop-up window that ask you to click Accept, in the top right, you can see the name of the Google account, click there and there is an option to log out. Then try again.

I have also read on a different thread that the problem can also be caused if you try to login with the Google account - again, use the link in the top right corner to check that Lightning is using the correct Google account.

Hope this helps