Web DAV folder 'X' cannot be searched: This error indicates that the specified mailbox folder cannot be processed. More specifically, the Web DAV API returned an HTTP 422 (unprocessable entity) error. In most cases, the folder cannot be accessed using a web client such as Outlook Web Access.
Check to see if the folder can be accessed using Outlook Web Access. Consider moving items from the folder to other folders and resubmitting your migration.