HTTP 400 Bad Request returned only when Java Applet connects to webserver

Experts,

I have a Java applet in my web application for rendering thumbnails. This is always working in most cases. However one of our customer reported that the Applet is not working. They deploy the web application on windows 2008 server. I check the logs and found that when the Java tries to connect to the IIS, I got the following HTTP 400 error:
Data: HTTP/1.1 400 Bad Request\r\nContent-Type: text/html\r\nDate: Thu, 03 Dec 2009 16:17:57 GMT\r\nConnection: close\r\nContent-Length: 20\r\n\r\n<h1>Bad Request</h1>
The url that the Java tried to access to is something like:
Data: GET /App/IrcXmlReq.aspx?Credentials=rxsizb2qnbf5ofddt0ki0zh&What=8&DataSource=CONTENT SERVER

I also tried to visit the url through IE, everything looks OK. I think there's something wrong in the IIS security configuration. Any help or suggestions will be appreciated. Thanks in advance!

private boolean tranSession()
  {
      boolean bRet = false;
      if(sCredentials == null || sTransessionURL == null)
        return false;
      try {
          String tranSessionURLReq = sTransessionURL + "?Credentials=" + sCredentials + "&What=8&DataSource=" + sDataSource;
          URL tranSessionURL = new URL(getCodeBase(), tranSessionURLReq);
          URLConnection con = tranSessionURL.openConnection();
          BufferedInputStream bis = new BufferedInputStream(con.getInputStream());
          ByteArrayOutputStream baos = new ByteArrayOutputStream();
          byte[] buf = new byte[BUF_SIZE];
          int bytesRead = 0;
          while ( (bytesRead = bis.read(buf)) != -1) {
            baos.write(buf, 0, bytesRead);
          }
          baos.flush();
          String sRet = baos.toString();
          baos.close();
          bis.close();
          if(sRet.contains("Success") )
            bRet = true;
      }
      catch(Exception e)
      {
      }
      return bRet;
  }

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