Efter few years of using 4.5.2 we're having problems with this error filling up the log very quickly:

At /xyz.aspx?ID=28 (Referred by: ): System.NullReferenceException: Object reference not set to an instance of an object. at umbraco.requestHandler..ctor(XmlDocument umbracoContent, String url) at umbraco.UmbracoDefault.Page_PreInit(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.EventHandler.Invoke(Object sender, EventArgs e) at System.Web.UI.Page.OnPreInit(EventArgs e) at System.Web.UI.Page.PerformPreInit() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

We're upgrading isnatllation now but not sure why is this hapening nor how to resolve it. Anybody has a clue where to start?