I cannot make any root operation under KDE Konsole (Fedora27 KDE):
su
Passwort:
su: Fehler bei Authentifizierung
sudo su
sudo: /usr/bin/sudo muss dem Benutzer mit UID 0 gehören und das »setuid«-Bit gesetzt haben
also any operation which requires root password

with Ctrl-Alt-F5 I can login as root

How can I fix it?

Yes... this was a decision taken some time ago by the devs
I dislike it but if you want to use KDE, it is what it is
My solution: install Krusader
Under the Tools menu there is an option to start as ROOT
It does any system/file changes as expected...

Fedora issue?

sudo su
works on Neon, OpenSuse, Kubuntu ...

When I used Fedora it was with Xfce...
currently use Neon User
and yes... "sudo su" works but that limited command line functionality is not always helpful, at least for me
which is why I often bail and resort to "krusader"
for now... it seems to bypass all the root limitations/frustrations that have been imposed on KDE