I took a moment to check all my settings and adjustments, but they all looked fine, so I did another focus and shot a triple burst. Here is what I got (AF-C set to FOCUS; spot meter; spot focus)
first
second
third
Then a loud metallic CLACK and everything froze. I had to pull the battery to clear it. This happened twice more, so I called it quits. When I got home I switched lens, both Tam and Nikon, and got it to happen two more times, but then I could not get it to happen again. I then tried to change to AF-S, and discovered I can't anymore.
I called Nikon and they agree that something is very wrong, quite possibly tied to my past accident, so it is going off to hospital. Better get those D5500 batteries charged up. . .