I set up Google Calendar to sync with Lightning in Thunderbird. It has worked well for the last few weeks. It has now suddenly stopped working. I have checked the CalDAV addresses and they are all still ok. I do not know if my recent addition of syncing my new Android phone with the Google Calendar has anything to do with it, or just a coincidence.
When Thunderbird tries to sync, it asks for the username and password. I enter these for the Google Calendar, and end up in a continuous circle.