After searching a bit more I found out that the %U parameter is part of the Desktop Entry Specification for .desktop files.
Recognized field codes:
%U A list of URLs. Each URL is passed as a separate argument to the executable program. Local files may either be passed as file: URLs or as file path.
It seems that the system settings program in KDE for creating startup entries from .desktop files doesn't take into account that desktop files could contain recognized field codes. Seems to be a KDE issue and have nothing to do with qbittorrent.