iCloud Calendar sync error 400 on OS X 10.10.2 I've been using Calendar with iCloud for a long while without any problems. Now, I have a permanent error displayed in Calendar:


  The request for account “iCloud” failed.
  
  The server responded with “400” to operation CalDAVAddSubscriptionCalendarQueueableOperation


I've searched around online and have tried all of the proposed solutions, but none of them seem to fix the problem. What should I try next?
Lets try this to Reset your iCal operations.

Back up calendars and Disable calendars in iCloud Preferences  :

Go to:


~/Library/Calendars
/Library/Preferences/com.apple.CalendarAgent
/Library/Preferences/com.apple.iCal.plist


Move all to the desktop.

Restart.