Clean installed 16.04. Previously running 14.04 mostly without problems. Box is an HP Z420 with a non-std display adapter (AMD Radeon R7 240). Lone Ubuntu only (no other OSes).
Problem: System Settings won't display its GUI, whether launched from launcher or command line. The unity-control-center process is running, but no GUI appears. No terminal output either if launched from command line.
apt-get install --reinstall ubuntu-desktop and
apt-get install --reinstall unity-control-center. Made no difference.
Looking for things to try or options for debugging or gathering a trace that might point out the problem.