Invalid viewstate and friends

 I cant wrap my head around this one!
 I receive about 50 of these errors everyday and i cant reproduce it.
Heres the stack trace:
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&a ...