FTP error: connection with the server was reset

We are running Windows 2003 server as a FTP server (no ISA). It worked for last year. Recently, we got many users and clients reports saying they receive “FTP Folder Error – An error occurred opening that folder on the FTP server. Make sure you have permission to access that folder – The connection with the server was reset”. The Event Viewer lists ID 1- User administrator at host x.x.x.x has timed-out after 120 seconds of inactivity. In the server, if I try to access the FTP in the FTP server, I receive this error: An error occurred opening that FTP server... Details: 200 type set to A. 227 Entering Passive Mode (172, 16 5, 2, 4, 250). 426 Connection closed; transfer aborted.

If I uncheck the Use Passive FTP in IE, I can access the FTP folders without any problems (details can be found on http://www.chicagotech.net/troubleshooting/eventid10.htm ). So it must be some settings in the server.