Nobody could answer this?!?
In the meantime, I escaped to the following work-around:
* dnf install @xfce-desktop
* Use 'startxfce4' instead of 'startkde' in ~/.Xclients
With this changes, the problem disappears (but I now log in into a xfce desktop, of
course), i.e. I could log in from a Win10 machine. Because of this, I still thinks it is a
kind of KDE issue. Perhaps selinux has its finger in the pie?