I'm a bit of a dope. Here's the story:
Two months ago a thin plastic bag got caught in the blades that dispense the ice into the tray.
To remove the plastic, I removed the add-in ice maker. When I replaced it, I pushed the plastic tube that feeds the water, just enough so it dripped a couple of drops into the tray.
I fixed the tube and problem solved.
There's some important advice here, but I don't know what it is!