ClassFormatError

What's the reason for the following error message of the Navigator?

# Applet debug level set to 9
# stopApplet: contextID=2 appletID=13800992 parentMWContext=13682640
# Stopping applet: pm, appletID=13800992, contextID=2
# destroyApplet: contextID=2 appletID=13800992
# Destroying applet: pm, appletID=13800992, contextID=2
#     total applets=1
#   destroying context for contextID 2
# Destroying applet context: http://Storm/Pm/pm.html, contextID=2
# Destroying AppletFrame without disposing: netscape.applet.DerivedAppletFrame[frame2,0,0,850x300,hidden,layout=java.awt.BorderLayout,resizable,title=]
#   frameMWContext=13682640
# Destroying all applets: http://Storm/Pm/pm.html, contextID=2
# initApplet: contextID=2 appletID=13771952 parentContext=13682640 frameContext=13682640
# New applet context: http://Storm/Pm/pm.html, contextID=2
# Initializing applet: pm.class, appletID=13771952, contextID=2
#   docURL=http://Storm/Pm/pm.html reloadClasses=true
#     total applets=2
#     New applet: 13771952 at http://Storm/Pm/ width=850 height=300 hspace=0 vspace=0 align=baseline codebase=http://Storm/Pm/ code=pm.class
# startApplet: contextID=2 appletID=13771952 newFrameMWContext=13682640
# Starting applet: pm, appletID=13771952, contextID=2
#   preventing reuse of netscape.applet.AppletClassLoader@800cda6
#   new class loader netscape.applet.AppletClassLoader@8008b86
#      context = netscape.applet.MozillaAppletContext@800d660
#      codebase = http://Storm/Pm/
# Loading class pm
# Fetching http://Storm/Pm/pm.class
# Destroying AppletFrame without disposing: netscape.applet.DerivedAppletFrame[frame2,0,0,850x300,hidden,layout=java.awt.BorderLayout,resizable,title=]
# Applet exception: error: java.lang.ClassFormatError: Wrong class name inside class file
java.lang.ClassFormatError: Wrong class name inside class file
  at java.lang.ClassLoader.defineClass(Compiled Code)
  at netscape.applet.AppletClassLoader.loadClass(Compiled Code)
  at netscape.applet.AppletClassLoader.findClass(Compiled Code)
  at netscape.applet.AppletClassLoader.loadClass1(Compiled Code)
* at netscape.applet.AppletClassLoader.loadClass(Compiled Code)
  at netscape.applet.AppletClassLoader.loadClass(Compiled Code)
  at netscape.applet.DerivedAppletFrame$LoadAppletEvent.dispatch(Compiled Code)
  at java.awt.EventDispatchThread$EventPump.dispatchEvents(Compiled Code)
  at java.awt.EventDispatchThread.run(Compiled Code)
  at netscape.applet.DerivedAppletFrame$AppletEventDispatchThread.run(Compiled Code)

Using JBuilder 2 and the AppletViewer the program runs well.
Maybe it's important that I have all class files in a directory called "pm" since all the files of the program should belong to the package pm.