Ping the back-end server to see if it is responding.
If it is not responding, determine why it is failing. When it is functioning again, proceed to the next step in this task.
Clear the CLD cache. See 12.5 Clearing the CLD Cache in Calendar Server Version 6.3.
If you are using the CLD cache option and you have updated a server name for an ics.conf parameter, you should clear the CLD cache to remove the server names. An out-of-date entry in the CLD cache can prevent a front-end server from establishing a connection to the correct back-end server or cause Calendar Server not to find a calendar after it have been moved.
Stop the server with the stop-cal command.
Restart Calendar Server using start-cal.