Having just installed Knoppix 5.1.1 from the CD, I also get this error.

This error does not happen with either a Debian installation from Knoppix 5.0.1, nor with a 'pure' Debian installation of 'Etch'.
In all cases I am running with the same version of KDE libraries (3.5.5a.dfsg.1-5), which at the time of this posting applies to both 'testing' and 'unstable'.

This suggests that it may be a problem specific to Knoppix 5.1.1 rather than KDE (and I can't find it listed in the KDE Bug Reports).

For security reasons, I prefer to retain the enforcement of case-by-case log-on as superuser(root). This works if you use the Gnome equivalent gksu instead of kdesu (apt-get install gksu).
For many applications you have to create custom KDE Buttons with executables such as the following examples (note the single quotes ' where applicable) :-
Code:
 gksu 'konqueror --profile filemanagement'
Code:
 gksu kuser
If anyone finds out how to get kdesu working, that's even better !