Follow up to 'java.lang.SecurityException: class “XYZ”'s signer information does not match signer information of other classes in the same package'

Question

I asked this question some time ago. I had an applet which contained classes in the default package and which is signed with an in-house certificate. Occasionally some users where getting the following error message:

java.lang.SecurityException: class "ParamSplitter"'s signer information does not
    match signer information of other classes in the same package
    at java.lang.ClassLoader.checkCerts(Unknown Source)
    at java.lang.ClassLoader.preDefineClass(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.defineClass(Unknown Source)
    at java.net.URLClassLoader.access$000(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at sun.applet.AppletClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.applet.AppletClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClassInternal(Unknown Source)
    at PortalLauncher.openFile(PortalLauncher.java:313)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.plugin.javascript.JSInvoke.invoke(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at sun.plugin.javascript.JSClassLoader.invoke(Unknown Source)
    at sun.plugin.com.MethodDispatcher.invoke(Unknown Source)
    at sun.plugin.com.DispatchImpl.invokeImpl(Unknown Source)
    at sun.plugin.com.DispatchImpl$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.plugin.com.DispatchImpl.invoke(Unknown Source)
java.lang.Exception: java.lang.SecurityException: class "ParamSplitter"'s signer
    information does not match signer information of other classes in the same package
    at sun.plugin.com.DispatchImpl.invokeImpl(Unknown Source)
    at sun.plugin.com.DispatchImpl$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at sun.plugin.com.DispatchImpl.invoke(Unknown Source)

I have since rebuilt the applet and no longer use the default package. The few classes I have are all in com.mycompanyname.portal. There is only one jar. I had hoped that the problem would be solved but it is not so.

There is always one other application in use on all the workstations which have this problem and this is IBM HostOnDemand. This might be a complete red herring but might mean something.

Any ideas? How might I find out which other classes are thought to be in the same package and have different signing information?