Server Error in '/ProgettoN' 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: 

The source code that generated this unhandled exception can only be shown when compiled in debug mode. To enable this, please follow one of the below steps, then request the URL:

1. Add a "Debug=true" directive at the top of the file that generated the error. Example:

  <%@ Page Language="C#" Debug="true" %>

or:

2) Add the following section to the configuration file of your application:

<configuration>
   <system.web>
       <compilation debug="true"/>
   </system.web>
</configuration>

Note that this second technique will cause all files within a given application to be compiled in debug mode. The first technique will cause only that particular file to be compiled in debug mode.

Important: Running applications in debug mode does incur a memory/performance overhead. You should make sure that an application has debugging disabled before deploying into production scenario.  

Stack Trace: 


[HttpException (0x80004005): Unable to validate data.]
   System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength) +397
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +293

[ViewStateException: Invalid viewstate. 
  Client IP: 127.0.0.1
  Port: 
  User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; SIMBAR Enabled; .NET CLR 2.0.50727)
  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) +169
   System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) +37
   System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +328
   System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) +30
   System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState) +52
   System.Web.UI.HiddenFieldPageStatePersister.Load() +256
   System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +137
   System.Web.UI.Page.LoadAllState() +102
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5916
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +188
   System.Web.UI.Page.ProcessRequest() +112
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +37
   System.Web.UI.Page.ProcessRequest(HttpContext context) +135
   ASP.utente_approvazione_aspx.ProcessRequest(HttpContext context) +29
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +401
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +117