It's cookie based. What normally happens is the first time through there's a check to see if Windows Authentication is turned on and the cookie is checked for an Undetermined status, if both are true then you're redirected to WindowsSignin.aspx and the cookie's status is set that it's using Windows Authentication. This keeps it from going into an infinite loop as the chunk of code is called quite often as part of the DNN process. The problem is (and why I suggest clearing your cookies/cache) is that even on logout the status is still set to Windows Authentication and if you visit the site before that portion of the cookie has expired (normally shouldn't be more than a couple of hours) it bypasses the redirection to WindowsSignin.aspx.

Now on to your experimenting.... I'm going to bet that you had Hide Login Controls checked. I had to use a little bit of code trickery for the Hide Controls portion. The wrapper for the login controls (which is part of the core) checks to see if each provider is enabled and if it is the login controls for that provider are loaded. What I had to do when Hide Controls is checked is to set the provider as not enabled but still store all of the pertinent AD information. So when you have Hide Controls checked and you disable the other providers then as far as DNN is concerned you don't have any providers enabled. And, really, you should have an emergency backdoor when you're using any of the providers.

Timster, I'm on my way out of town in the next 1/2 hour but will try to get a beta up this evening.

I have the exactly same issue. Have you been able to find a solution to the problem Mike?