caldav account has no folder tree & does not appear as a calendar

I’ve been using em client for my family (4 installs) for a short time, as a client and email its great.
However, I have a problem that affects all the familes installations.
I use Zimbra as my groupware back end and this works well with the IOS/android devices etc with caldav for calendars so the caldav url and system is working.
I create an account using the auto setup, it sets up imap, smtp and caldav/carddav, I enter all the details and they are accepted.
Whilst the imap folders are visible and work, there are no calendar folders available anywhere for use.
If I try to select a default calendar folder, the only folder tree made available are the local folders - these dont work as then I get the error - this folder is not related to your account etc.
I have also created a dedicated caldav account on the machines, same url, etc, again no folder tree is created - which makes em client unusable as a contact and calendar client (same issue for contacts folders as well)
I’m sure its something I’m doing but cannot work out what it is any suggestions gratefully grabbed.

That can happen when the CalDAV URL is not correct. The login to the service appears correct, but the calendar is never populated.

Unfortunately there are often different URLs for Android/iOS and Windows clients, so you can’t just copy it from one to the other. And even among different Windows applications, the same URL does not always work.

Spot on, Gary.
After some brute force changes through a set of different common URI layouts, all works perfectly.
I used - https://servername/dav/useremailaddress/Calendar/
Many thanks.