Hi,
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:
https://github.com/nextcloud/server/issues/10134
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).
Solved.
Anyway, thought I’d share my results for you all.
Kevin