Cloudd wants to use the "Local items" keychain I opened my MBP and certainly the system gives the following warning "Cloudd wants to use the "Local items" keychain" and asks me for my keychain-password. After cancelling this warning, it kept coming back, over and over again. After a forced quit and a system reboot the warning was gone.

I never had any idea why my system whould ask for my password at that time, was someone uninvited trying to access my computer? Who or what is Cloudd (double-d)?
It's a private framework made by Apple to interface with iCloud documents and files.

It's safe to allow and there's a possibility of keychain corruption if it won't stick (and since you wouldn't normally get that warning unless the automated process to save your iCloud password has hit a bump like having keychain items deleted or expiring)

From https://discussions.apple.com/thread/6684809


  Back up all data before proceeding.  
  
  Launch the Keychain Access application in any of the following ways:
  ☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
  ☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
  ☞ Open LaunchPad. Click Utilities, then Keychain Access in the icon grid.  
  
  Select the login keychain from the list on the left side of the Keychain Access window. If your default keychain has a different name, select that.  
  
  If the lock icon in the top left corner of the window shows that the keychain is locked, click to unlock it. You'll be prompted for the keychain password, which is the same as your login password, unless you've changed it.  
  
  Right-click or control-click the login entry in the list. From the menu that pops up, select
  Change Settings for Keychain "login"  
  
  In the sheet that opens, uncheck both boxes, if not already unchecked.
  From the menu bar, select
  Keychain Access ▹ Preferences ▹ First Aid  
  
  If the box marked
  Keep login keychain unlocked
  is not checked, check it.
  Select
            Keychain Access ▹ Keychain First Aid
  from the menu bar and repair the keychain.  
  
  Quit Keychain Access.