iCloud Calendar sync error 400 on OS X 10.10.2
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.