Update:
Before completing a ticket I decided to try one more test. I started another new game, but this time just clicked "quick start". I then immediately tried to save and then reload the game. No problem!
I then went back to several of my save games that had been giving the problem (namely, an attempt to load another game while a game was in progress). After about a hafl dozen attempts without any CTDs I have to conclude that something had glitched and got sorted by loading the "quickstart". (I did not reboot or do any other prep.)
I could speculated that it had something to do with creating custom races before I started play. But that is sheer guesswork at the moment. Since I can't produce the symptoms at this point I am going to close this unless it reappears.
In particular, as I get a chance, I will be creating more custom races and doing some testing, but I am not very confident I can reproduce the symptoms, so at this point I will chalk it up to a one time glitch.