New to Java

I have an applet which connects to a servlet.I have just upgraded from Java 1.5_06 to Java 1.6_027.
I have a login.html which opens up when i hit the url http://localhost:80/test/testlogin.html.
The IIS 7 opens this page and then when I provide login credentials it should connect to the testServlet but I get
an IOException at the line
ObjectOutputStream out = new ObjectOutputStream(conn.getOutputStream());

saying 'Server returned code 500 for the url http://localhost:80/test/testServlet.class

I am using the following lines of code

conn = servletURL.openConnection();
conn.setDoOutput(true);
conn.setUseCaches(false);
conn.setRequestProperty("Content-Type", "application/octet-stream");
ObjectOutputStream out = new ObjectOutputStream(conn.getOutputStream());
This code use to work fine with Java 1.5 but it doesnt work after the upgrade.
I am not sure if it is a Java problem or IIS or Weblogic.
Please help
Thanks,