When I click after about 15 minutes on a page I wrote I recieve the
following error:

Server Error in '/' Application.
--------------------------------------------------------------------------------

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) +2549692
  System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) +214

[ViewStateException: Invalid viewstate.
    Client IP: 72.81.21.124
    Port: 50536
    User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET
CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30)
    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.ObjectStateFormatter.Deserialize(String inputString) +251
  System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String
serializedState) +4
  System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter,
String serializedState) +37
  System.Web.UI.HiddenFieldPageStatePersister.Load() +220
  System.Web.UI.Page.LoadPageStateFromPersistenceMedium() +83
  System.Web.UI.Page.LoadAllState() +35
  System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6953
  System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint,
Boolean includeStagesAfterAsyncPoint) +154
  System.Web.UI.Page.ProcessRequest() +86
  System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +18
  System.Web.UI.Page.ProcessRequest(HttpContext context) +49
  ASP.default_aspx.ProcessRequest(HttpContext context) +29
  System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
+154
  System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously) +64

--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET
Version:2.0.50727.210

I have no idea why this happens, and am unfamiliar with viewstate MAC. Can
anyone give me a starting point? Thanks.