adding a caldav https path within Outlook 2007

I have a CALDAV address to a https server.
My Outlook 2007 let's me add an address beginning with caldav://server
..... without any problems.

Though I would like to add a calendar that has an address beginning
with https:// server; Outlook returns an error message and aborts.
I used the https address on my iPhone. It asked me for my login
credentials of the server - and now it works (on the iPhone) but not
on Outlook.

Anyone an idea, how I can fix the required authentication?

Thx, Daniel