In the file lang/en/backup.php, we can't find the definition of the string 'questioncannotberestored' that appears on line 710 of the file backup/util/dbops/restore_dbops.class.php :

$errors[] = get_string('questioncannotberestored', 'backup', $question);

But perhaps this is the string 'questionegorycannotberestored' that should be renamed to 'questioncannotberestored':

$string['questionegorycannotberestored'] = 'The questions "{$a->name}" cannot be created by restore';