I've noticed massive problems with latest ktorrent svn builds, that started quite some time ago, but never investigated on that issue.
The problem is showing in a way that ktorrent will display a (!) on a torrent. Specifically because announce will fail - "receiving corrupt data"
or "Connection to host tracker.some_private_address.net is broken."...

I've tried multiple tests with other clients on same trackers and it is working fine. Now I've disabled kio usage for doing tracker request and after
restarting ktorrent 6x times in a row, there isn't a single problem reported on same torrents. There needs to be done more testing, but I'm pretty
sure this is the cullprit, using kio for announces would be problematic almost on every startup.

What could be the problem here? I've set in kde systemsettings that I use direct internet connection.

The problem is trackers sending back broken HTTP which is badly interpreted by KIO's http code. Seeing that it is really not a KIO bug, I added an alternative HTTP implementation, which is more lenient.