java and signed jars on Mac

I have a thick Java client that is launched as jnlp. The jars are all signed.

this application works on all OSes except on Mac OSX 10.5.8.

Using Safari or FireFox, when I launch the app, I get the following error:

java.lang.NullPointerException
at java.util.jar.JarVerifier.mapSignersToCodeSource(JarVerifier.java:496)
at java.util.jar.JarVerifier.mapSignersToCodeSources(JarVerifier.java:508)
at java.util.jar.JarVerifier.getCodeSources(JarVerifier.java:826)
at java.util.jar.JarFile.getCodeSources(JarFile.java:613)
at java.util.jar.JavaUtilJarAccessImpl.getCodeSources(JavaUtilJarAccessImpl.java:25)
at com.sun.deploy.cache.DeployCacheJarAccessImpl.getCodeSources(DeployCacheJarAccessImpl.java:60)
at com.sun.javaws.security.SigningInfo.getCommonCodeSignersForJar(SigningInfo.java:382)
at com.sun.javaws.security.SigningInfo.check(SigningInfo.java:291)
at com.sun.javaws.LaunchDownload.checkSignedResourcesHelper(LaunchDownload.java:1498)
at com.sun.javaws.LaunchDownload.checkSignedResources(LaunchDownload.java:1268)
at com.sun.javaws.Launcher.prepareResources(Launcher.java:1222)
at com.sun.javaws.Launcher.prepareAllResources(Launcher.java:624)
at com.sun.javaws.Launcher.prepareToLaunch(Launcher.java:332)
at com.sun.javaws.Launcher.prepareToLaunch(Launcher.java:204)
at com.sun.javaws.Launcher.launch(Launcher.java:121)
at com.sun.javaws.Main.launchApp(Main.java:418)
at com.sun.javaws.Main.continueInSecureThread(Main.java:255)
at com.sun.javaws.Main$1.run(Main.java:115)
at java.lang.Thread.run(Thread.java:613)

Is this a function of the browser settings or the Java configuration on this machine? As I said the jars are signed and verified on all other OS/platforms except this one OS/machine.

Any ideas on this