We recommend replacing older versions of clients with the latest versions of qbittorrent, transmission or rtorrent (if using a seedbox). These older versions either do not support https trackers or do not support larger size pieces. By keeping the old versions, you risk not getting peers or that the torrent file will not be recognized as valid. And to the question "which version of utorrent to use" the answer is "none, install qbittorrent". If you still do not want to change utorrent to qbittorrent, increase the number of active torrents in the settings, it will be your responsibility when sooner or later it will crash. Users who use utorrent do not receive support for any customer issues, but only the recommendation to use one of the clients recommended above.