As I supposed, the error was eventually found in the UserLogin.aspx.cs file in that incorrect fields or properties were causing the error. The user comments were appreciated and helped me locate the issue although the Classic/Integrated mode issue was not the cause after all. To complicate matters, in my Global.Application Errors I was redirecting to UserLogin and hence the circular errors. Changing it to redirect to Defaults.aspx also helped as the underlying error was in part caused by incorrect cookies. In short - the answer to my question is it loads the next redirected/default page starting from that pages construction which is before any other events including the page.Preinit event.