this problem make me crazy! i have asp.net website it raise periodically this error (in IE8):

    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)
    ,...

or (in IE6)

System.FormatException: Invalid length for a Base-64 char array.
   at System.Convert.FromBase64String(String s)
   at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)

or (in IE7)

System.FormatException: Invalid character in a Base-64 string.
   at System.Convert.FromBase64String(String s)
   at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)

i set enableViewStateMac in web.config to false and defined machinekey in my web.config and defined UTF-8 encoding for every page but i received this errors.

do you have any solution?
best regards