I realise that this is an old thread, but I hope that my post can help at least one person.
I am now using Thunderbird 91.6.1 although I imagine that what I'm describing here happens in older versions too.
1. When adding a Google calendar, the username should be the full email address
XYZ@gmail.com and not just ZYZ
2. When I gave only ZYZ as the username, Thunderbird was able to read the calendar (maybe because I was already subscribed to another calendar of
XYZ@gmail.com), but could not write to it.
I got an error code MODIFICATION_FAILED
I also got the triangle with an exclamation mark (next to the calendar name) and a message "The Google calendar ABC is momentarily not available"
If Thunderbird could give a more helpful message, such as "Google did not accept the username and password" (or maybe "Stop being an idiot"
) that would be great.