This worked for me after the initial install, I could add other users' calendars into my account. BUT, I wanted to use https to access zimbra, therefor I ran zmctl mixed to make it possible (I hope this was the right way).
Now, when I try to add their URL to the new calendar box I get:
"The following URL is not currently reachable". I have tried http and https url's.
If i manually enter the url into the browser, pass the auth ok then get the calendar.ics download.
im a bit stumped, can anyone offer any support?