_webform_safe_name return empty in the download file has title in greek.

In order to reproduce the error you have to setup greek as a language, create a form and the go to results -> download the file will have only the extension like "xls", "tcsv", etc. I'm not sure why we don't use just (probably for security issues?).

$export_name = $node->title;

In webform/includes/webform.report.inc:

function webform_results_download($node, $export_info) {
  global $user;
  $export_name = _webform_safe_name($node->title);

  if (empty($export_name)) {
      $export_name  = t('Untitled'); // << the fix
  }