Blank LogOn view in MVC app

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ASP.NET

MVC

On Friday I gave a demo of an MVC application using the Authorize attribute on my JobCardController, and all was well. This morning, when I attempt an unauthorized action, I still get redirected to the LogOn view, using the below URL, but I appear to have what my reading tells me is called a White Screen of Death, i.e. nothing but white background and no actions invoked.

http://localhost:34054/Account/LogOn?ReturnUrl=%2fJobCard

ADDENDUM: I forgot to mention that I have moved the Account models and services out into my DataObjects and Services projects, but I had done this last week already without this problem.
I'm not asking what is wrong, but what are the common faults that could cause something like this and how can I start debugging this mess?