The “Microsoft fix” (ctrl-alt-del and wait for the reboot to complete) took care of the problem. Figure!
No, it’s not a headless machine and a mate-session was running when the problem occurred. The system is on the SD card that the RPi4 boot from.
I was starting to think that this could have been an installation problem. Now I’m pretty sure that it was. Anyway, it’s working again and the simple fix is ‘chmod u+s NX/bin/nxexec’ and reboot.
Jim