16.04 System Settings not displaying

Question

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.

I've tried 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.

Solution

Of course I figure it out less than 24hrs after posting.

I found that lsusb was hanging. I had a DELL S2340T monitor plugged into one of the USB3 ports (the monitor is a hub and also supports touch via USB...DisplayLink, etc.). Unplugging that fixed lsusb, allowed unity-control-center to display, and also fixed a hang-at-logout problem.

No idea whether the problem is specific to the USB3 on this box or the just the monitor. Don't care either, as I don't need any of the features of USB3 or USB from the monitor. Just glad 16.04 is working now...