You didn’t post the exact error you’re seeing.
More than likely there is a supported format issue, I doubt the problem is the magnet technology.
One way to verify this is if as you say the torrent file can be downloaded with a different torrent client, open up the file in a text editor and compare it with some other torrent file which is no problem using either client.
It may be that ktorrent might actually be the better performing client in this case, particularly if you’re getting your torrent files from something like oldpiratebay.org which is only a demo of a very, very dead thepiratebay.
TSU