The installed name of tool kioslave conflicts with the related tool from KDE4 installations. It should be renamed to kioslave5. Installing the tool in libexec does not help on Windows where libexec dir is set to bin too.
Not an issue anymore as all old KDe4 stuff is long gone.
This is still an issue with cross compiled KF5 windows builds at https://build.opensuse.org/package/show/home:rhabacker:branches:windows:mingw:win32:KF547/mingw32-kio, which could be coinstalled with the KDE4 counterpart The related patch is located at https://build.opensuse.org/package/view_file/home:rhabacker:branches:windows:mingw:win32:KF547/mingw32-kio/0001-Install-kioslave-as-kioslave5-to-avoid-conflicts-wit.patch?expand=1
https://phabricator.kde.org/D17650
I see /usr/lib64/libexec/kf5/kioslave5 on my system, so it's been fixed at some point.