After I use a nested master page ,I face the following problem :

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

and sometimes

Invalid related information

[ViewStateException: Invalid viewstate. 
    Client IP: 127.0.0.1
    Port: 
    User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:17.0) Gecko/20100101 Firefox/17.0
    ViewState: 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...]

[HttpException (0x80004005): Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.]
   System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) +116
   System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +13
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +220
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +5
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +37
   System.Web.UI.HiddenFieldPageStatePersister.Load() +202
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +106
   System.Web.UI.Page.LoadAllState() +43
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3905
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +262
   System.Web.UI.Page.ProcessRequest() +79
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +109
   ASP.frm_items2_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\a961040e\19f53d4c\App_Web_frm_items2.aspx.cdcab7d2.nkfrbsfi.0.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +399
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +76

I use this Tool

to fix this problem but in vain !!

I don't know even the root cause of this problem !!