[kronolith] Error retrieving free/busy information

[L.W. van Braam van Vloten]

I have installed Horde Groupware 1.0 and I am trying  to get the  
free/busy functionality in Kronolith to work.

I realize this question has been discussed before in this list, but so  
far I could not find an answer to my question. So I will try again :)

Horde is installed in a secure apache server. The PHP openssl  
extension is installed and working.
I have created a shared address book that is to be used by kronolith  
to retrieve the free/busy url's.
Authenticated users have all rights to this address book.
The free/busy url's all look like: https://<web IP  
address>/horde/kronolith/fb.php?u=<username>. When I use this url  
directly in my browser the free/busy information is provided as  
expected. The <web IP address> is the external IP address of my ADSL  
modem; all https traffic is NATted to the actual server which has a  
different IP address.

Yet every time I try to schedule an appointment, I  get the following  
error for all attendees:

Error retrieving free/busy information for <e-mail address of user>:  
No free/busy information found at the free/busy url of <e-mail address  
of user>

So I am wondering:
 From which location (client or server) is this url actually used?  
Address 'localhost' also doesn't work. I figured the problem might be  
that I use HTTP Basic Authentication for all addresses under the  
secure server. If the application uses the URL locally, is there a way  
to provide the authentication information automatically? (I already  
tried the https://user:password@<IP address>/... which didn't work)

Any suggestions?

Kind regards,
Lucas