caldav client unknown to mdaemon

I am trying to install a graphic interface caldavzap (open source web client) on a server of the company (Windows Server 2003), so that it can show the calendar located in another domain. https://www.inf-it.com/open-source/clients/caldavzap

The server where the resource or the calendar is the MDaemon mail server and I'm not the administrator. The location of the web client does not seem complicated, but I always get an unauthorized 401 and cross domain error messages. Caldavzap tells me that there may also be problems with the TLS. On the other hand, calendars can be viewed and created from mozila thunderbird (with plugin lighting). So I guess could see it from the customer caldavzap. My question is: Is it the customer's problem? Do I need to access the messaging server to allow access to my client that is on another server?