
Recent versions of Apple Calendar require an SSL certificate installed on the Zulu server.
Your calendars must be published: How To Publish Calendars. Zulu must be installed: Zulu - Installation. 4.1.3 Option 3: Allow Unauthenticated Requests (Calendars can be viewed without credentials). 4.1.2 Option 2: Add The Default FileMaker Certificate To Your Trust Store. 4.1.1 Option 1: Install a Valid SSL Certificate.
4.1 Unable to verify account name or password (SSL Certificate Requirement for Apple Calendars). 3.2.2 Adding a CalDAV account without AutoDiscovery. 3.2.1 Adding a CalDAV account with AutoDiscovery. 3.1.2 Adding a CalDAV Account Without AutoDiscovery. 3.1.1 Adding a CalDAV Account With AutoDiscovery.