I'm trying Ktorrent since a few weeks and encounter nearly permanent connexion errors to trackers. Half of the tries for the announces ends in time-out. Apparently, this applies only to UDP trackers (udp://tracker...). The consequence is a very slow download (<10% of the normal) with highs and lows clearly visible on the graphs. I tried on two other computers and after updating to 4.3.1 with the same results. Two others bittorrent client on the same computers does their job normally on the same torrents.
By the way, I noticed that it was impossible to remove or modify a tracker from the original list of a torrent and that unckecking one does not stop ktorrent trying to contact it. On the other hand, adding a new tracker to the list does not work as it seem that ktorrent never try to contact it.
After a lot of tries and check, I'm quite sure that the problem comes from ktorrent...