What makes a Java Applet Jar Invalid?

I have an issue with an applet. The issue has occured from more than one computer. The applet was last deployed in April. It was working from then.What can we look for that would invalidate the ability to read a jar file?

network: Downloading resource: https://[path and file name removed for privacy].jar
      Content-Length: -1
      Content-Encoding: null
java.io.IOException: Invalid jar file
      at com.sun.deploy.net.DownloadEngine.getJarFileWithoutCache(Unknown Source)
      at com.sun.deploy.net.DownloadEngine.getJarFileWithoutCache(Unknown Source)
      at com.sun.deploy.net.DownloadEngine.downloadJarWithoutCache(Unknown Source)
      at sun.plugin.PluginURLJarFileCallBack$2.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at sun.plugin.PluginURLJarFileCallBack.retrieve(Unknown Source)
      at sun.net.www.protocol.jar.URLJarFile.retrieve(Unknown Source)
      at sun.net.www.protocol.jar.URLJarFile.getJarFile(Unknown Source)
      at sun.net.www.protocol.jar.JarFileFactory.get(Unknown Source)
      at sun.net.www.protocol.jar.JarURLConnection.connect(Unknown Source)
      at sun.plugin.net.protocol.jar.CachedJarURLConnection.connect(Unknown Source)
      at sun.plugin.net.protocol.jar.CachedJarURLConnection.getJarFileInternal(Unknown Source)
      at sun.plugin.net.protocol.jar.CachedJarURLConnection.getJarFile(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath$JarLoader.getJarFile(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath$JarLoader.access$1000(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath$JarLoader$1.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at com.sun.deploy.security.DeployURLClassPath$JarLoader.ensureOpen(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath$JarLoader.<init>(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath$3.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at com.sun.deploy.security.DeployURLClassPath.getLoader(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath.getLoader(Unknown Source)
      at com.sun.deploy.security.DeployURLClassPath.getResource(Unknown Source)
      at sun.plugin2.applet.Plugin2ClassLoader$2.run(Unknown Source)
      at java.security.AccessController.doPrivileged(Native Method)
      at sun.plugin2.applet.Plugin2ClassLoader.findClassHelper(Unknown Source)
      at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
      at sun.plugin2.applet.Plugin2ClassLoader.loadClass0(Unknown Source)
      at sun.plugin2.applet.Plugin2ClassLoader.loadClass(Unknown Source)
      at sun.plugin2.applet.Plugin2ClassLoader.loadClass(Unknown Source)
      at java.lang.ClassLoader.loadClass(Unknown Source)
      at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unknown Source)
      at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
      at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
      at java.lang.Thread.run(Unknown Source)
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Last i heard, with current JREs, all applets must be signed now
mccarlIT Business Systems Analyst / Software DeveloperCommented:
Make sure that the JAR is being served properly too. Put the address (https://[path and file name removed for privacy].jar) into a normal browser address bar and make sure that is downloads the JAR file properly. Check that the downloaded jar file is correct.
jvmnjAuthor Commented:
Thanks mccarl.

I have done that it downloads and was able to view the files in a decompiler. I can confirm that the same jar file loads using the same client browser in 2 other deployed environments. What do you all think could be wrong with this environment?
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You could well have different runtimes - did you test them?
jvmnjAuthor Commented:
Thanks CEHJ.

But the the same applet jar, ran by the same user, from the same desktop (same jre) works in 2 environments. But in the quality environment it reads invalid jar. Any other thoughts?
jvmnjAuthor Commented:
Gonna close this. It turns out that we were accessing the application through our siteminder url. Siteminder was configured to ignore the path that the applet resides. So in order for the applet to load you have to use the direct websphere app url.

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jvmnjAuthor Commented:
I found the answer on my own
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