Click on "Start Tray App" in the sidebar of Hudson.
Java Webstart opens, asks if the certificate should be trusted (seems to be a self-signed certificate), if the content may be executed.

I agree to certificate and web-start content. Starts loading, and aborts with this exception:

com.sun.deploy.net.JARSigningException: Unsignierter Eintrag in Ressource: http://hostname/hudson/plugin/hudsontrayapp/jdic.jar gefunden
at com.sun.javaws.security.SigningInfo.getCommonCodeSignersForJar(SigningInfo.java:382)
at com.sun.javaws.security.SigningInfo.check(SigningInfo.java:256)
at com.sun.javaws.LaunchDownload.checkSignedResourcesHelper(LaunchDownload.java:1384)
at com.sun.javaws.LaunchDownload.checkSignedResources(LaunchDownload.java:1170)
at com.sun.javaws.Launcher.prepareLaunchFile(Launcher.java:1033)
at com.sun.javaws.Launcher.prepareToLaunch(Launcher.java:309)
at com.sun.javaws.Launcher.prepareToLaunch(Launcher.java:186)
at com.sun.javaws.Launcher.launch(Launcher.java:107)
at com.sun.javaws.Main.launchApp(Main.java:405)
at com.sun.javaws.Main.continueInSecureThread(Main.java:252)
at com.sun.javaws.Main$1.run(Main.java:111)
at java.lang.Thread.run(Thread.java:613)

It would be cool if that could be fixed. Please do not hesitate to write me a message if you need more information.