After latest libKF5KIOCore.so.5.65.0 update my KTorrent has stopped to work via HTTP Proxy.
I've created a Bug here: https://bugs.kde.org/show_bug.cgi?id=414346 to make it more googleable if someone else faces with this problem now.
The bug in the line here: https://github.com/KDE/kio/commit/0911763b8f19c9fee3083e7f09382e1cde957723#diff-7d1e19d94808019256c1784b3dfc6801R2163
The method returns QVariant with type QByteArray that can't be directly converted to QStringList. It should be converted to QString and splitted.