Ping the back-end server to see if it is responding.
If it is responding, go to step 3. If it is not responding, determine why it is failing, and when it is functioning again, proceed to
Clear the CLD cache. See Clearing the CLD Cache.
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 An out-of-date entry in the CLD cache can prevent a front-end server from establishing a connection to the correct back-end server or cause Calendar Server not to find a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.
Restart Calendar Server.