When I add a magnet link to Ktorrent, from Firefox, Ktorrent freezes soon after the download starts if Firefox is still running.
If I close Firefox, the freezing doesn’t happen.
I encountered the same problem on Leap 42.2 and 42.3.
My current Leap15 installation is a clean one, while the home partition is old, legacy from Leap 42.3.
Funny thing, some weeks ago I noticed the freezing problem had disappeared, while I was still running 42.3.
I had the same problem with KTorrent. In addition to freezing upon opening a magnet link, it would also crash at random if I had more than 5 or 6 torrents in the system. They did not necessarily have to be active, just having them in the program would cause it to crash. I never found a solution, I just uninstalled KTorrent and started using qbittorrent. I haven’t had a problem with it. I would just find another torrent program if I were you.