You're probably right, but it is ugly to have both the web page title and the icon at the bottom showing a cryptic string.
Also, I noticed that if you do click on the icon (or the words "USER_ROOT.ics"), you're prompted to authenticate, after which you get a 404 error.
It looks like the icon is "left over" from the view that you get if you view (with authentication) https://zimbraserver.domain.com/home/<username>/Calendar.html
This suggests that it should really point to https://zimbraserver.domain.com/home/<username>?fmt=ifb instead of what it does now,
https://zimbraserver.domain.com/home/USER_ROOT.ics