Any ideas why this is happening?  This is not a webfarm. 

InnerException: 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.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: 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.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. ---> System.Web.UI.ViewStateException: Invalid viewstate. Client IP: 178.93.137.128Port: 4851User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Crazy Browser 2.0.0 Beta 1; .NET CLR 1.0.3705; .NET CLR 1.1.4322)ViewState: 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...--- End of inner exception stack trace ---at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError)at System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState)at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString)at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState)at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter formatter, String serializedState)at System.Web.UI.HiddenFieldPageStatePersister.Load()at System.Web.UI.Page.LoadPageStateFromPersistenceMedium()at System.Web.UI.Page.LoadAllState()at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)--- End of inner exception stack trace ---
Source:

For me this turned out to be due to missing namespaces section underneath (see in italics below).  Some upgrade along the way took those out.  Seems to be running fine now.

<pages validateRequest="false" enableViewStateMac="true" enableEventValidation="false">

<namespaces>

<add namespace="System.ComponentModel" />

<add namespace="System.Data" />

<add namespace="System.Data.SqlClient" />

<add namespace="System.Drawing" />

<add namespace="Microsoft.VisualBasic" />

<add namespace="System.Globalization" />

<add namespace="DotNetNuke.Services.Localization" />

<add namespace="DotNetNuke.Entities.Users" />

<add namespace="DotNetNuke" />

<add namespace="DotNetNuke.Common" />

<add namespace="DotNetNuke.Data" />

<add namespace="DotNetNuke.Framework" />

<add namespace="DotNetNuke.Modules" />

<add namespace="DotNetNuke.Security" />

<add namespace="DotNetNuke.Services" />

<add namespace="DotNetNuke.UI" />

<add namespace="DotNetNuke.Entities.Portals" />

<add namespace="DotNetNuke.Common.Utilities" />

<add namespace="DotNetNuke.Services.Exceptions" />

<add namespace="DotNetNuke.Entities.Tabs" />

</namespaces>