I saw a jar-hell related bug in another ElasticSearch forum, so I assumed it was a related bug and didn't look carefully enough at the error message.
I've removed ant-javafx.jar from the JDK, and it now works. But it honestly seems like an odd thing to have to do--alter some files from the default installation of the Oracle JDK. But for now, I don't use JavaFX anywhere, so hopefully this won't cause other problems.
Thanks for you help.