Strange behaviour which I believe is a recent change, although to be honest, I don't know whether it's KStars or the UbuntuMate. The fits file open dialog in KStars does not fully open or populate.
Other software's file open dialogs are working correctly. Double clicking on a fits file in the OS GUI does launch KStars and correctly display the image.
Using the debugger in QTCreator I noticed that after opening the FileOpen dialog, KStars seemed to be rendering the HiPS overlay (forever, more or less) when attempting to open a Fits Image. Hence the dialog somehow stalled without fully populating its elements. I turned off HiPS rendering and tried a File Open. It worked. After restarting KStars, I turned on the HiPS DSS Coloured overlay again and things seem to be working fine now. No hung file open dialog.
Confused, but happy.
I wasn't able to deduce exactly what the problem was.
I should note that on my Odroid-N2, the initial display of the DSS overlay is garbled, but it quickly renders itself properly after a few seconds.