Tried adding a calendar following your CalDav instructions for Thunderbird; however Thunderbird interface has changed and whether I try to use:

https://calendar.zoho.com/caldav

or

https://calendar.zoho.com/caldav/{CalendarID}/events

I get the message (after entering password): Could not find calendars at this location.  Please check your settings.

Does the CalDav no longer work with Thunderbird?

Thanks,

Amit