Try this:
<sessionState mode="InProc" customProvider="DefaultSessionProvider" timeout="60">
<providers>
<add name="DefaultSessionProvider" type="System.Web.Providers.DefaultSessionStateProvider, System.Web.Providers, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" connectionStringName="DefaultConnection" />
</providers>
</sessionState>
Off course you need to have a connection named DefaultConnection.