I just upgraded from 10.7 server to 10.8 server.
sudo serveradmin fullstatus calendar
says its state is 'running'.
When I create a OSX server account under system preferences -- mail, contacts & calendars, it recognizes calendars, but it says that "Calendar couldn't log in to the CalDAV server".
The calendar service seems qute basic, and I don't know what else to change. The ports are open. Other services (VPN, file sharing) work fine.
On the apple support forum I found an entry related to keychain access, I deleted pretty much everything related to my server. Did not help.