Java io IOException: Applet/class class

Hello,
This is very strange.. I have a php-web application that uses the PopupMenuApplet.class

This works great .. on some systems and on some clients. From server A it works on client x and y. From server B it works on client x but not on client y. Clients are IE or FireFox on Window systems...

The one that gives a problem say:
java.lang.ClassNotFoundException: PopupMenuApplet.class
at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.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)
Caused by: java.io.IOException: open HTTP connection failed:http://192.168.250.4/VECTST/PopupMenuApplet/class.class
at sun.plugin2.applet.Applet2ClassLoader.getBytes(Unknown Source)
at sun.plugin2.applet.Applet2ClassLoader.access$000(Unknown Source)
at sun.plugin2.applet.Applet2ClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
... 7 more