It's hard to tell exactly what might have been changed that is causing this. Do you have any continuous integration testing showing when the regression has been introduced exactly?
In any case, AFAICS the ClassCastException there comes from a failure in calling Class::asSubclass(Handler.class) on your handler class. Can you try debugging a bit and checking which classloaders are involved there? I assume your handler is properly implementing javax.xml.ws.handler.Handler, so there's likely something weird with the classloader(s) that loaded your handler and Handler, right?