View State problem

Hi all,

My client has experienced the ff. error. He got this error when closing the IE and logging in again.

----
Server Error in 'webvendor' Application

The View State is invalid for this page and might be corrupted.

Description: An unhandled exception occurred during the execution of the
current web request.
Please review the stack trace for more information about the error where it
originated in the
code.

Exception Details: System.Web.HttpException: The View State is invalid fo
this page
and might be corrupted.


----
I have this is my page:

--
<%@ OutputCache Duration="300" VaryByParam="none"%>
<%@ Page Language="vb" AutoEventWireup="false" Codebehind="dynamiccal.aspx.vb" Inherits="webvendor.dynamiccal" Strict="True" EnableSessionState="ReadOnly" EnableViewState="false"%>
--

I have no server.transfer method. Does anybody know how to resolve this?


Thanks in advance.

I also would like to add that not all users are experiencing this kind of error. Some of my users are accessing the system through dial-up. Does this error has something to do with user's ISP or user's firewall?

Any ideas?