I've been experiencing random hangs using KTorrent 4.3.1 (libktorrent 1.3.1) when opening magnet links. Upon opening, if things go south, KTorrent will become unresponsive for the most part and given a few minutes, the entire computer will become extremely sluggish. When it begins, the log file ~/.kde4/share/apps/ktorrent/log will fill up within seconds and start rotating the logs filling all of 10 of them in a minute or so. CPU usage maxes out around one core at 100% and memory usage starts growing fast and after few minutes KTorrent ends up having eaten almost all available memory. I don't know whether or not it would've kept going at the swap next, at this point I finally killed it from the console.

I managed to repeat it so that I could kill it fast enough to get the beginning of a log before it was overrun, the last few lines are here:

Sun Jul 20 16:06:38 2014: Metadata download, requesting 0
Sun Jul 20 16:06:38 2014: Metadata download, requesting 0
Sun Jul 20 16:06:38 2014: Metadata download, requesting 0
Sun Jul 20 16:06:38 2014: UTP: established connection with 84.106.102.46
Sun Jul 20 16:06:38 2014: Authentication to 85.23.58.79 : failure
Sun Jul 20 16:06:38 2014: Initiating connection to 85.23.58.79 via (UTP)
Sun Jul 20 16:06:38 2014: Authentication(S) to 84.106.102.46 : ok
Sun Jul 20 16:06:38 2014: UTP: established connection with 192.168.1.1
Sun Jul 20 16:06:38 2014: Metadata download, dowloaded 0
Sun Jul 20 16:06:38 2014: Metadata downloaded
Sun Jul 20 16:06:38 2014: Metadata download, dowloaded 0
Sun Jul 20 16:06:38 2014: Metadata download, received piece 0 has the wrong size
Sun Jul 20 16:06:38 2014: Metadata download, requesting 0
Sun Jul 20 16:06:38 2014: Metadata download, requesting 1
Sun Jul 20 16:06:38 2014: Metadata download, requesting 2
Sun Jul 20 16:06:38 2014: Metadata download, requesting 3

After that the last entry repeats while incrementing the number up to 102222, then starts again at zero and goes and goes and goes.

Any ideas?

The exact same thing started happening to me today. Happened with different magnet links, so it's not specific to one download.

A few days ago this was working perfectly fine.

I'm using the same version of KTorrent and libktorrent as you.

I was able to fix this now be emptying ~/.kde4/share/apps/ktorrent/magnets. That file contained information about a different download that I had finished downloading about a week earlier. I'm not sure if this was really the culprit, but at least now KTorrent works again without hanging after a few minutes.