ASP.NET MVC Windows Security issue

I am suddenly getting a Windows Security dialog when requesting:

http://mydomain/Reports/

This happens after I have moved an ASP.NET MVC application from Server 2008 to Server 2008 R2.

This page (like others) are access controlled using Windows Form authentication. The other pages work correctly, just this page is giving me hassles - and only on this server.

Even signing in with my domain credentials fails; after the third failed login a blank page appears.

I have disabled Windows Authentication in IIS and enable Form Authentication.

The request executes:

    public ActionResult Index()
    {
        return View();
    }

The index.aspx page associated to his page contains static HTML.

Any ideas wha开发者_Go百科t might be causing this?

Update to comments and questions:

• Site (mydomain) is running as a web site on its own.
• SSRS is installed on the machine but not part of this web site.
• Changing the controller name to ReportController instead of ReportsController causes it to work. Something is processing Reports first, where can I find it? I don't see any Reports folder or web sites on the server...

When upgrading sql server, often it adds a virtual directory or virtual application in iis called Reports and ReportManager. Just went through this, so it's worth checking out.

Check the permissions on the folder or file, they are probably not set correctly so IIS is trying to get a valid set of credentials to continue.

Also, is that Reports folder running, say, SSRS?