Fails to access web calendar

[GuruX]

Zimbra Desktop seam to be a really good mail/calendar client if it works. I can't get it to communicate/synchronize with my web calendar via CalDAV. Mail via IMAP is ok.
Using the same setup that I use in Tunderbird/Lightning and working there is not working with Zimbra. Error message in something like: The specified URL cant be reached". Our serverhost is Kerio but it works with other open standards as Thunderbird/Lightning.
My question: Is it supposed to work? - I get split indications from zimbra webpage. Any idea what is wrong? When configuring calendar Im not asked about password which feels a little strange...