Webstart of Scrumworks desktop version ends in NullPointerException

Edit Subject

since I upgraded to java version 1.6.0_20, I can not run the desktop client anymore. I have tried un-installing and re-installing this java version but keep getting the same problem: Unable to launch the application.

Details contains the following trace:
java.lang.NullPointerException
at java.util.jar.JarVerifier.mapSignersToCodeSource(Unknown Source)
at java.util.jar.JarVerifier.mapSignersToCodeSources(Unknown Source)
at java.util.jar.JarVerifier.getCodeSources(Unknown Source)
at java.util.jar.JarFile.getCodeSources(Unknown Source)
at java.util.jar.JavaUtilJarAccessImpl.getCodeSources(Unknown Source)
at com.sun.deploy.cache.DeployCacheJarAccessImpl.getCodeSources(Unknown Source)
at com.sun.javaws.security.SigningInfo.getCommonCodeSignersForJar(Unknown Source)
at com.sun.javaws.security.SigningInfo.check(Unknown Source)
at com.sun.javaws.LaunchDownload.checkSignedResourcesHelper(Unknown Source)
at com.sun.javaws.LaunchDownload.checkSignedResources(Unknown Source)
at com.sun.javaws.Launcher.prepareResources(Unknown Source)
at com.sun.javaws.Launcher.prepareAllResources(Unknown Source)
at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
at com.sun.javaws.Launcher.prepareToLaunch(Unknown Source)
at com.sun.javaws.Launcher.launch(Unknown Source)
at com.sun.javaws.Main.launchApp(Unknown Source)
at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
at com.sun.javaws.Main$1.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

Collegues of me are using the same version but do not seem to have any problems with the jnlp webstart. Can you please advice me on how to get this solved?

Kind regards,
Ad