Comment # 8 on bug 1170443 from Wolfgang Bauer

(In reply to Frank Kruger from comment #7)
> (In reply to Rainer Klier from comment #6)
> > to be sure that it is not the case, that somehow the root password has
> > changed, i did "passwd" as root user and set it to the value it should
> > currently be.
> > 
> > but it didn't change anything.
> > 
> > "su -" also fails.
> > 
> > for some unknown reasons any attempt to use "su" does not work, so it is a
> > problem with su and not kdesu.
> 
> So, the issue is not due to kdesu. Could you please run 'sudo bash'?

And therefore "KDE Workspace (Plasma)" is certainly the wrong component here
too.
Maybe "Basesystem is better"...