[HttpException (0x80004005): Unable to validate data.]

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i have a form that its action redirects to another page!
when i do that i get the error message :
"[HttpException (0x80004005): Unable to validate data.]
System.Web.Configuration.MachineKey.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +195
System.Web.UI.LosFormatter.Deserialize(String input) +60

[HttpException (0x80004005): Authentication of viewstate failed. 1) If this is a cluster, edit <machineKey> configuration so all servers use the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. 2) Viewstate can only be posted back to the same page. 3) The viewstate for this page might be corrupted.]
System.Web.UI.LosFormatter.Deserialize(String input) +118
System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +101

[HttpException (0x80004005): Invalid_Viewstate
Client IP: 212.179.136.176
Port: 3989
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.0.3705)
ViewState: dDwtMTk0NjkzNDQyOztsPE5vO05vO3llczt5ZXM7Pj4MpTht/fewdSwo20GOp36ENrJKxg==
Http-Referer: <!-- m --><a class="postlink" href="http://www.anak-hamazganim.co.il/AcRepair1.aspx">http://www.anak-hamazganim.co.il/AcRepair1.aspx</a><!-- m -->
Path: /servvars1.aspx.]
System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +442
System.Web.UI.Page.LoadPageViewState() +18
System.Web.UI.Page.ProcessRequestMain() +447
"

what can i do?
thanks i nadvance
peleg