I'm getting 100+ errors per day on my website with System.Web.HttpException: Invalid viewstate.

The website is asp.net 3.5 running on iis6 , not running in a web-garden/web-farm , single server.

Here are a few sample errors.

Machine: ML Framework Version: 2.0.50727.3603 Assembly Version: 6.5.3664.33889 
Source: http://www.domain.com/WebResource.axd?d=z5VmXXoSLLpQHoPictureAlert 
Exception: System.Web.HttpException: Invalid viewstate. at System.Web.UI.Page.DecryptStringWithIV(String s, IVType ivType) at System.Web.Handlers.AssemblyResourceLoader.System.Web.IHttpHandler.ProcessRequest(HttpContext context) 
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() 
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) 

Machine: MLFramework Version: 2.0.50727.3603 Assembly Version: 6.5.3664.33889 
Source: http://www.mydomain.com/ScriptResource.axd?d=SE0Ej7OlEAx91j2Cjv_6KkRPplqT-5wB4M7CZPdGdGn3LahLwqlRPApUcdxBsbFXYHZ91Q76FHAHWgHs8SmOC4zemr7
siym0QY0rF3XtJTu%3C/a%3E%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%3Ca%20id= 
Exception: System.Web.HttpException: Invalid viewstate. at System.Web.UI.Page.DecryptStringWithIV(String s, IVType ivType) at 
System.Web.UI.Page.DecryptString(String s) at System.Web.Handlers.ScriptResourceHandler.DecryptParameter(NameValueCollection queryString) at 
System.Web.Handlers.ScriptResourceHandler.ProcessRequestInternal(HttpResponse response, NameValueCollection queryString, 
VirtualFileReader fileReader) at System.Web.Handlers.ScriptResourceHandler.ProcessRequest(HttpContext context) at 
System.Web.Handlers.ScriptResourceHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext context) at 
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at 
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) 

1. i already tried wraping all inline javascript with //<![CDATA[ //]]>
2. i already set enableViewStateMac to false.

From looking at all the errors guessing out of the "d" paramter it seems to focus on a single usercontrol on my website. in this control i change the visiblity of div's + text in the usercontrol OnPreRender function.

protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            PreparePage();
        }

Can the errors be related to the usercontrol behavioral?

thanks!