That works indeed, weird stuff.
I added permission java.security.AllPermission; in my java.security file and that also worked around the problem.
A senior Java developer that just walked past my desk and looked at the code of the AppletLoader thought that it might be because of the usage of:
Class.forName(getParameter("al_main"));
he recommended to instead use:
Thread.currentThread().getContextClassLoader().loadClass(getParameter("al_main"));
Don't ask me though because I am not that familiar with the inner workings of Java but it might be an idea
