I couldn't ready your screen shot very well, what type of error is it?  It almost appears as if it is a page not found error, but I cannot tell from that picture.

Well, it's good to hear your problem is gone, just wish we knew what it was!  *grin*

Well, then I might still be able to help you find the cause of that error...

Here's the case: I'm having the exact same problem Namret described in this topic. Difference here is that I'm running DNN 5.2.3, with AD provider version 5.0.2 (current latest stable). All of it running on Windows Server 2003, with IIS6 and MSSQL 2005 Express.

What's making this so very odd and undeclarable, is that when I call the ActiveDirectory/WindowsSignin.aspx-URL directly using another hostname (of course configured in DNN's alias list and in my local hosts-file), it does work! Both hostnames are redirecting to the very same IP-address, so no difference there. Also there are no host headers configured in IIS and, even if I do set one, it doesn't help.

Nothing usable is getting logged anywhere, I have checked DNN's event logs as well as my IIS-logs and all I can get, is from the latter; IIS is returning a status 401.1, and right after that a 401.2... All Internet Explorer (7) shows, is the common "Internet Explorer cannot display the webpage"-message; completely useless information.. Disabling the "Friendly error messages"-setting in IE doesn't change any of that.

I have tried Mike's "Website Authentication Testing File", that I stumbled upon during one of my many internet quests; it's working flawlessly, no matter what hostname I use when calling it. Therefore I recon this must be something, somewhere between DNN/the AD-provider and IIS...

Any suggestions would be very welcome as I am eager to get this fixed as soon as possible, as more and more users are asking for automatic login.