[Error Code] 0x80070057 [Explanation] 
This error can safely be ignored, as the tool will continue to migrate the objects without the missing field. On some of the events we're moving we are unable to locate a property in the object that we are trying to import. A potential cause of this is an older version of Exchange or Outlook that doesn't use all of the latest fields. If you are seeing problems, try to create a new admin account in Exchange. If your already created a new admin, and using a profile to do the migration, make sure Cached Exchange mode is disabled and that you clicked "Always allow" on any autodiscovery messageboxes if they pop up.