Are you using a CDN or CloudFlare?
Could you try re-uploading the 3.5.1 version of class.file.php. Then find this around line 80:
$tokenData = $db->getRow('SELECT id, user_id, ip_address, file_id, download_speed, max_threads FROM download_token WHERE file_id = ' . $db->escape($this->id) . '...