Website always occur the following error Failed to load viewstate
<p>Recently, I don't why our website always occur the following error,but before I never receive the same errors. 
The  request IP is <code>91.201.66.76</code> , I don't know my hosting meet attack.</p>

<p>Error Message: Failed to load viewstate.  The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request.  For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.</p>

<pre><code>Request URL: http://www.hicalc.com/SignUp.aspx

Request IP: 91.201.66.76    Language:     Logined User: Anonymous

Beijing Time: 3/17/2011 2:57:41 PM ( UTC+8 )     Web Server Local Time: 3/17/2011 12:57:41 AM

HttpUnhandledException: "Exception of type 'System.Web.HttpUnhandledException' was thrown."
   at System.Web.UI.Page.HandleError(Exception e)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.signup_aspx.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&amp; completedSynchronously)

HttpException: "Failed to load viewstate.  The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request.  For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request."
   at System.Web.UI.Control.LoadViewStateRecursive(Object savedState)
   at System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState)
   at System.Web.UI.Control.LoadViewStateRecursive(Object savedState)
   at System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState)
   at System.Web.UI.Control.LoadViewStateRecursive(Object savedState)
   at System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState)
   at System.Web.UI.Control.LoadViewStateRecursive(Object savedState)
   at System.Web.UI.Page.LoadAllState()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
</code></pre>
