I am sorry lxnay, my information was not clear and led to some misunderstanding.
My problem is, that I cannot open neither a new terminal session nor a X-Sessions using the CTRL+ALT+F-Key-combination.
CTRL+ALT+F-Key has no effect during a KDE-Session and also not on the KDM-Login-Screen.
mkdir -p /var/tmp had no effect.
Graphical Sessions in Konsole and Yakuake are working perfectly.