Validation of viewstate MAC failed with nested master page

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After I use a nested master page ,I face the following problem :\[quote\] 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.\[/quote\]and sometimes \[quote\] Invalid related information\[/quote\]\[code\][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\[/code\]I use this Toolto fix this problem but in vain !!I don't know even the root cause of this problem !!