Meanwhile I tried to compile KStars from source with Qt 5.9.9 on Ubuntu 16.04 with success. The display bug is gone, BUT the responsivity bug persists! I suppose it is a KStars/indi bug: The option buttons for the serial/ethernet connection are all dead*. Shall I open a ticket in KStars bugtracking systems?
*) Surprisingly they are all working when I'm debugging an INDIdriver with QtCreator.