Wierd problem

Below is a very simple example servlet, which generates a form, with 1 input field, and a submit button. Is re-posts to itself.
If I run using a local Tomcat/Apache server it works fine, I can submit the form as many times as I want.
However, when I run via our IPlanet webserver, the first submit works, but the second submit give a error message:
Bad request
Your browser sent a query this server could not understand.
The code is:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class testform extends HttpServlet {

public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html");
PrintWriter out = response.getWriter();

out.println("<HTML><HEAD><TITLE>Test form</TITLE></HEAD><BODY>");

out.println("<FORM METHOD='POST' ACTION='testform'>");

out.println("NAME:<INPUT TYPE='TEXT' NAME='name' VALUE=''>");
out.println("<INPUT TYPE='SUBMIT' NAME='SUBMIT' VALUE='SUBMIT'>");

out.println("</FORM></BODY></HTML>");
}

public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}