Note that if you’re using Thunderbird 64, I couldn’t get it to connect to CalDav, and it was returning a 503 Error in the Console.
I found this article however and was able to resolve it:
Go to Thunderbird Options, Advanced Pane, General Tab
At the bottom is a ‘Config Editor’ button
Accept the risk (if applicable)
Search for ‘network.cookie.same-site.enabled’
Set to False
Restart Thunderbird (I also had to re-subscribe to the calendar).
Anyway, thought I’d share my results for you all.