java.lang.ClassCastException: class jdk.proxy2.$Proxy8 cannot be cast to...

[apb2006]

I want to run the socket server standalone. I have downloaded org.eclipse.lemminx-uber.jar 0.31.0

java -cp org.eclipse.lemminx-uber.jar org.eclipse.lemminx.XMLServerSocketLauncher

From a local web page I do

   const webSocket = new WebSocket("ws://localhost:5008");

At which point lemminx crashes

   Exception in thread "main" java.lang.ClassCastException: class jdk.proxy2.$Proxy8 cannot be cast to class org.eclipse.lemminx.customservice.XMLLanguageClientAPI 
(jdk.proxy2.$Proxy8 is in module jdk.proxy2 of loader 'app';
 org.eclipse.lemminx.customservice.XMLLanguageClientAPI is in unnamed module of loader 'app')
        at org.eclipse.lemminx.XMLLanguageServer.setClient(XMLLanguageServer.java:267)
        at org.eclipse.lemminx.XMLServerSocketLauncher.launch(XMLServerSocketLauncher.java:60)
        at org.eclipse.lemminx.XMLServerSocketLauncher.main(XMLServerSocketLauncher.java:36)

java -version
openjdk version "17.0.16" 2025-07-15
OpenJDK Runtime Environment Temurin-17.0.16+8 (build 17.0.16+8)
OpenJDK 64-Bit Server VM Temurin-17.0.16+8 (build 17.0.16+8, mixed mode, sharing)

What am I doing wrong?