I upgraded from 6.2.9 to 7.2 and then 7.2.1, friendly urls set to advanced, been through the process of setting up the custom 404 page (AUM_ErrorPage404 setting in portalsettings). 

The problem I have, is that I get a pageload exception every time I navigate to a non existent page. This happens whether or not the AUM_ErrorPage404 setting is present :

DotNetNuke.Services.Exceptions.PageLoadException: Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the <configuration>\<system.web>\<httpModules> section in the application configuration. ---> System.Web.HttpException: Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the <configuration>\<system.web>\<httpModules> section in the application configuration. at System.Web.UI.Page.get_Session() at DotNetNuke.Framework.CachePageStatePersister.Save() at System.Web.UI.Page.SaveAllState() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---

I've been through the web.config the settings are correct :

    <modules runAllManagedModulesForAllRequests="true">
      <remove name="Session" />
      <add name="Session" type="System.Web.SessionState.SessionStateModule"/>      

     <httpModules>
      <add name="Session" type="System.Web.SessionState.SessionStateModule"  />

...

    <pages validateRequest="false" enableViewStateMac="true" enableEventValidation="false" viewStateEncryptionMode="Always" enableSessionState="true" >

My 404 page just has a html module with nothing special. 

I've spent a day on this now and gotten nowhere. I have removed any unused modules, updated to the latest on the third party modules, spent ages diffing my web.config with a fresh install... all to no avail. Anyone have any idea what could cause this?  

Regards

Vincent.