Thanks for your answer. The setup is really simple, this is what I described above. On the server side: Ordroid XU4, Ubuntu 16.04, a standard VNC server (TightVNC, TigerVNC or TurboVNC). On the client side: any standard client on Windows, Android, ... (tightVNC, VNC Viewer). In all cases and combinations I get the same error, with and without encryption. This error only shows up with KStars and not with any other program used on the Odroid through VNC (PHD2, CCDciel, Siril, ...) - and as I said KStars works perfectly well when launched from the connected display on :0
The VNC server is launched typically with the command "vncserver :1 -geometry 1600x900 -depth 24".
It seems something wrong happens when using KStars / Qt on the X11 display :1
I will try an ssh -X session from an other Linux platform to give it a test.
Thanks again.

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