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.