I was getting similar strange behaviour.
Windows (XP), Java 1.5.0.x (x=3,4,5).
I was changing installations and modifying the JWS prefs, but I found that the cache must have been shadowed somewhere.
I was running against a newly installed Servoy Developer, but even thought the cache had been deleted the ‘old’ servoy_client.jnlp descriptor seemed to be sticky.
So, I re-configured and then restarted the box.
Problem gone.
This is the first time in a longtime that I’ve had to re-start the box for a Java application to work !