Repeated requests to update icloud preferances
I faced the same problem on my MacBook Pro (mid 2012) and I fixed it by following the guidelines mentioned here: http://www.rawinfopages.com/mac/content/keep-being-asked-your-icloud-password


  "Go to System Preferences, iCloud and clear all the ticks so that you
  are no longer syncing anything to iCloud. You should also go into
  every Apple app (FaceTime, Calendar, Messages, iTunes, Reminders,
  Notes and any other app you can think of), check the preferences in
  each app and if you are signed in to iCloud then sign out.
  
  After disabling all the iCloud sync options in System Preferences and
  every single Apple app, return to iCloud in System Preferences and
  tick all the boxes again to start syncing once more. You might also
  need to sign in again with FaceTime and the other apps you signed out
  of."