My ktorrent seems to be not working anymore. All torrents have the status "Queued for downloading" for ages.
Would that be a problem in cofigurations?

Increase the amount of torrents being run simulataneously by the queue manager. (Settings -> Configure KTorrent -> Queue Manager)

Thanks. It is working now. I've set to "no limit".
Is that a good choice or is there a reason I should choose a smaller number?

It depends on how many torrents you are running. If you run to many torrents at the same time, you limit the number of connections per torrent (there is a limit on the total amount of network connections), which can negatively impact the speed.