The SQL port defaults to 6597 unless changed via the SQL_PORT keyword in ctsrvr.cfg. This port must be unused and available or you may see "SQL subsystem initialization failed" in the CTSTATUS.FCS file. You may edit the SQL port to a different value.
Be sure to verify that the server is not trying to set up an IPv6 port when it should be using an IPv4 port (see IPv6 Support).