When I try to start a torrent that is already completed and has the current status "not started" I get the error message "Cannot start more than 0 downloads. Go to Settings -> Configure KTorrent, if you want to change the limit."
Changing the number of allowed downloads doesnt't help.
I'm using ktorrent 1.2rc1 (32bit) on Suse 10.0

The inclusion of the QueueManager fixed this bug, so if you upgrade tot he latest SVN, it will be gone.