Running a java applet in authenticated folder

In one website I am working on we're using a java applet in 2 places: the 1st one in a public area where it works just fine and one in a protected folder where it just doesn't work. The protection is performed with forms authentication. The error is shown below. Any clue ?

java.lang.ClassFormatError: Incompatible magic value 218774561 in class file activeup/activeupload/UploadApplet
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at s ...