Error en la validación de la MAC del estado de visualización.

Hola amigos,
Estaba trabajando en un proyecto. Terminé ese proyecto cuando abro este proyecto cuatro días después, aparece un error.

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. 

En este proyecto nunca utilicé ningún cifrado para la validación. cuando entro en la red no veo ningún sitio, me pide que configure

<pages enableEventValidation="false" viewStateEncryptionMode ="Never" />

en el archivo webconfig. Incluso después de hacer esto, sigo recibiendo el error. Este error aparece cuando estaba recuperando datos de la base de datos y mostrándolos en gridview. ¿Qué debería querer hacer?
debajo de mi archivo webconfig.

<?xml version="1.0"?>
<configuration>
    <configSections>
        <sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
            <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
                <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
                <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
                    <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
                    <section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>
                    <section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false"/>

                </sectionGroup>
            </sectionGroup>
        </sectionGroup>
    </configSections>
    <system.web>
        <pages>
            <controls>
                <add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            </controls>
            <tagMapping>
                <add tagType="System.Web.UI.WebControls.CompareValidator" mappedTagType="System.Web.UI.Compatibility.CompareValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add tagType="System.Web.UI.WebControls.CustomValidator" mappedTagType="System.Web.UI.Compatibility.CustomValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add tagType="System.Web.UI.WebControls.RangeValidator" mappedTagType="System.Web.UI.Compatibility.RangeValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add tagType="System.Web.UI.WebControls.RegularExpressionValidator" mappedTagType="System.Web.UI.Compatibility.RegularExpressionValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add tagType="System.Web.UI.WebControls.RequiredFieldValidator" mappedTagType="System.Web.UI.Compatibility.RequiredFieldValidator, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add tagType="System.Web.UI.WebControls.ValidationSummary" mappedTagType="System.Web.UI.Compatibility.ValidationSummary, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            </tagMapping>
        </pages>
        
        <compilation debug="true">
            <assemblies>
                <add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
                <add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/>
                <add assembly="System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
                <add assembly="MySql.Data, Version=6.2.3.0, Culture=neutral, PublicKeyToken=C5687FC88969C44D"/></assemblies>
        </compilation>
        <httpHandlers>
            <remove verb="*" path="*.asmx"/>
            <add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            <add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false"/>
        </httpHandlers>
        <httpModules>
            <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        </httpModules>
    </system.web>
    <system.web.extensions>
        <scripting>
            <webServices>
                <!-- Uncomment this line to customize maxJsonLength and add a custom converter -->
                <!--
      <jsonSerialization maxJsonLength="500">
        <converters>
          <add name="ConvertMe" type="Acme.SubAcme.ConvertMeTypeConverter"/>
        </converters>
      </jsonSerialization>
      -->
                
            </webServices>
            <!--
      <scriptResourceHandler enableCompression="true" enableCaching="true" />
      -->
        </scripting>
    </system.web.extensions>
    <system.webServer>
        <validation validateIntegratedModeConfiguration="false"/>
        <modules>
            <add name="ScriptModule" preCondition="integratedMode" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        </modules>
        <handlers>
            <remove name="WebServiceHandlerFactory-ISAPI-2.0"/>
            <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
            <add name="ScriptResource" verb="GET" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        </handlers>
    </system.webServer>
</configuration>