Killing the Python.exe processes did not work the first or second time.
I remembered that since I last used the PLC Editor simulator, I had installed some software (Apache Tomcat, ...) that installed some services and set them to Auto start. I stopped those services, and set them to Manual start.
The PLC Editor gave the same error when simulating the ladder-logic project that worked a few months ago. When it spawned a second Python.exe process, both Python.exe processes did not seem to consume very much CPU, but things kept running for a very long time until the compilation completed and the error showed up in the log.
I restarted the PLC Editor again, then restarted and opened the Hello World project. The simulator worked on that. Now the PLC Editor works on the ladder-logic project from a few weeks ago, as well as the newest one that I created.
It is almost as if the errors from my mis-configured ladder-logic changed some sort of state that was remembered, perhaps in a file somewhere.