Validation of viewstate MAC failed.

"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."

Set EnableViewStateMac="false" for Specific page then set on page directory

<%@ PageLanguage="C#"AutoEventWireup="true"CodeFile="Dashboard.aspx.cs"Inherits="_Default"  EnableViewStateMac="false" %>

For all page then this will set on webconfig file

<page EnableViewStateMac="false">
page >