You guys are great! I really appreciate everyone taking the time to reply.
I installed FFmpeg right after re-installing OT. That's why I had the choice of MP4 in the Rendering output settings. So, that wasn't the problem.
BUT - I've figured this out: I was trying repeatedly with one scene I especially wanted to export. Turns out it had a screwed-up attempt at adding FX in the Schematic area. I was experimenting with FX for the first time and balled it all up, I still don't get how to use the FX I want to try. And I don't know how to fix the problem I accidentally created. But the mistake I made there led the scene through a bogus dead-end output.
I've now exported other scenes with zero problems because none had my botched FX attempts screwing up the output.
So - setting aside FX for now, more pressing things to do. But wanted you to know what my problem was.
Thanks again!