I finally got it working, but I don't know how. A couple things I learned in the process:
1) Safe Mode with Networking does work over wifi, but you have to click on the Network icon (globe) in the lower right corner and connect to your network before entering your password.
2) MSO Repair would not run in Safe Mode with Networking - it appears that you cannot start MS Click-To-Run service in that mode.

The last thing I did before the problem disappeared was to run MSO Quick Repair with a wired connection to the internet (after a normal windows startup). But I don't believe this was the solution, because the Quick Repair had completed successfully several times with a wifi connection without resolving the problem.


Anyway, thanks again for all your help.