Validation of viewstate MAC failed

Server Error in '/[APPLICATION]' Application.

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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.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: 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.

Source Error:

[No relevant source lines]

Source File: c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\[APPLICATION]\8a1e65ba\14dfca6d\App_Web_td0fs2md.2.cs    Line: 0

Stack Trace:

[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) +198 System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose) +432 System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter2.Deserialize(String serializedState, Purpose purpose) +8 System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose) +40 System.Web.UI.HiddenFieldPageStatePersister.Load() +248 System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +88 System.Web.UI.Page.LoadAllState() +36 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6704 System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +245 System.Web.UI.Page.ProcessRequest() +72 System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21 System.Web.UI.Page.ProcessRequest(HttpContext context) +58 ASP.login_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\[APPLICATION]\8a1e65ba\14dfca6d\App_Web_td0fs2md.2.cs:0 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +341 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

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Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18044