This error occurs when inheritable permissions are not enabled on a mailbox. You will see this error when trying to remove an ActiveSync Device:
To fix this, first enable ‘Advanced Features’ in ADUC:
Then open the user’s properties and browse to the Security tab. Click ‘Advanced’ and then place a check in the box labeled ‘Include inheritable permissions from this object’s parent’. Click OK twice to apply. Wait for AD to replicate to your GC’s then retry the action in Exchange. It will now remove the device successfully.