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.
It is present in Ubuntu 16.04 AND 18.04 and I could trace a really strange behaviour in both: As I stated before the "Connection Mode" buttons of the INDIDrivers is always working under debugging. The spooky thing is, that after the very first start of EKOS (after a new startup of the OS) the buttons are working too! But if you start Ekos once again these buttons get unresponsive!
It there a crack out there, who has more insight? I haven't got a clue.
I had mentionned about unresponsive buttons in another thread (#49172) and what I had found was that if I deleted the xml files related to the driver (in ~/.indi) the buttons would be responsive again, until the xml files were created again.
@knro: First, I didn't believe it either! But yes, it's true: Starting the OS (16.04 or 18.04) from zero, does it!
But only for the first invocation of the driver (by EKOS or by Device Manager). All subsequent calls render the Connection Mode buttons unresponsive.
@G_Gagnon: I spotted this thread and I can confirm your note. There has to be a relation! Alas, until now I could not spot the part of the source code, where the problem is located. Besides, I think that it is not a practicable way to erase the XML-File every time you want to change something.