To enable the client connection to Kerio MailServer, it is necessary to set correct URL address for the connection.
If there is no important reason for manual configuration, configure the CalDAV account with the iCal Config Tool (section 35.2 CalDAV account in Apple iCal). In addition to the account, this tool will configure also the Directory Utility where it sets Kerio MailServer as an Open Directory server. This setting will make delegation of calendars fully functioning. Without this setting, subscription of delegated folders only is available.