I am trying to sync my Zimbra calendar with Outlook 2007 and by that also with my MDA.

The new Outlook supports iCal which should make it possible.
To configure it in Outlook I go to tools->account settings->internet calendars->new
Then I use "https://mail.domain.com:8443/home/username/Calendar" as URL.
After clicking OK I get the message: "Cannot verify or add the internet calendar in Outlook. Verify the link is a valid calendar link.".
I never get a login prompt in Outlook.

Using exactly the same URL in Mozilla Sunbird and even Windows Vista Calendar it works perfectly.

Does anyone have an idea what goes wrong in Outlook 2007?

Using Zimbra Open source edition 4.5.6