I managed to find a workaround.
I noticed that during my Debian system upgrade the mesa packages were updated from 17.1.5-1 to 17.2.1-2. If I roll them back to 17.1.5-1 then everything works again.
This leads me to conclude that the changes between mesa 17.1.5 and mesa 17.2.1 are breaking GLX on NoMachine Workstation virtual sessions.