Now i installed Kubuntu 12.04.2 LTS and made the upgrade to Kubutu 13.04.
After the first reboot the plasma-desktop ran without any crashes but began than to crash again.
Therefore i deleted the plasma config files "~/.kde/share/config/plasm*" as described by Adam Maćkowiak (admc) and since then it runs stable (so far).
Now i installed Kubuntu 12.04.2 LTS and made the upgrade to Kubutu 13.04. share/config/ plasm*" as described by Adam Maćkowiak (admc) and since then it runs stable (so far).
After the first reboot the plasma-desktop ran without any crashes but began than to crash again.
Therefore i deleted the plasma config files "~/.kde/