MVC Forms LoginUrl is incorrect

This is a known issue. I had the same problem with my custom authorize attribute. I found the solution somewhere on the net, can't remember where. Just add this to appSettings in your web.config

<add key="loginUrl" value="~/Account/LogOn" />

Note: This works with MVC 3, I didn't try it with previous versions.

EDIT: Found it mentioned in release notes, but it seems that they've changed the setting name and forgot to update release notes for RTM version.

I ran into a similar problem sometime ago. After a few months I discovered the root of the problem: I had added a 'deployable dependency' on 'ASP.NET Web Pages with Razor Syntax'. This adds a reference to: WebMatrix.Data.dll This assembly has a class with a static constructor that does the following:

static FormsAuthenticationSettings()
{
 FormsAuthenticationSettings.LoginUrlKey = "loginUrl";
 FormsAuthenticationSettings.DefaultLoginUrl = "~/Account/Login";
} 

Check if you are referencing this dll.

frennky's answer helped me get to this. I needed all of these in my web.config:

<appSettings>
  <add key="loginUrl" value="~/Authentication/LogOn" />
</appSettings>

<system.web>
  <authentication mode="Forms">
    <forms loginUrl="~/Authentication/LogOn" timeout="2880"></forms>
   </authentication>
   <authorization>
     <deny users="?"/>
   </authorization>
</system.web>