BTW, a translation of uckelman's reply:
>>> Does VASSAL work with Java 9? 10? 11?
Java changed a few things in version 9, which broke programs written for Java 8. Deleting the xercesImpl.jar file gets things going again.
BTW, it's unlikely that this is the only issue. The warning messages that Philippe reported at the start of this thread were just warnings i.e. the reflection operations they were complaining about were being allowed to work properly, Java was just telling you that will not be the case in the future, which means that the problems he was seeing were being caused by something else :-/
>>> Why are you seeing illegal access warnings with Java 9 and later?
There were bugs in Java itself, they added some hacks to get around them, which unfortunately required them to do some things you're not really supposed to do, and it's come back to bite them in the ass. Not really their fault, sometimes you have to do these things :-/
>>> Why is our minimum requirement still Java 5?
This sounds like it's on Apple. I do mostly Python these days, and Apple ships an ancient version of Python by default, so it's entirely believable they do/did the same thing with Java.