If a Web server sends more data than the Content-Length header specifies, the Proxy server may determine that this is a violation of the use of the Content-Length header. Proxy will then close the port that is being used to download the data. If the port was opened by Internet Explorer, using the 'Proxy Connection:Keep-Alive' header, the Proxy server will set the TCP FIN Flag in the final response frame on the port.
The Keep-Alive session will cause Proxy to re-use the TCP socket for subsequent HTTP requests. This causes Proxy to issue a TCP Reset on the port, because it previously closed the port.
To resolve the issue, use Network Monitor to verify that the Web server is sending more data than the Content-Length header indicates. Asks the Web site to correct the problem.