I doubt it's to do with JApplet.
It's too long ago for me to remember what, if any, is the significance to the applet of your codebase parameter (as opposed to attribute), but I'm a bit suspicious of that and also of the code attribute: does signed code have to be in a package, or is it OK for it to be in the default package? Certainly, the applet class name "newposting" is contrary to convention, though that wouldn't cause an error in itself.. So I'd be tempted to refactor to put your classes in a package (i.e. include the package statement in your .java files).
What does your web server access log say? Lot's of 200 and 304 status codes, or 404?