I brought the issue to GOG support, and they recommended doing a Windows 10 Clean Boot Mode start.
1. Type MSC into the Search box.
2. Start the System Configuration app.
3. Select Diagnostic Startup.
4. Restart computer.
This immediately fixed the problem for me. And when I went back to the System Configuation app and reset it to Normal startup and rebooted, the game continued to work.
I'll have to see if something stops the game again, maybe then I'll have a better idea what it is.
Post edited January 16, 2020 by FDPercival