After a long time I decided to change the linux distribution. I did so after the problems with the apt-get and update of the debian linux distribution (sid). I downloaded the iso image of ububntu linux version 6.10, burned the image and installed.
The installation was completed without any problem. Ok, I am not a big fan of the Gnome environment, but it looked very good and was very user-friendly. Anyway.. I wanted my KDE desktop environment that I am more used to.
One issue I discovered was, that I could not specify the proxy server within the graphical package-manager tool. I made the workaround and added the proxy support to /etc/apt/apt.conf.d/01ubuntu manually. The apt-get install kde was a piece of cake then.