ACS installed but MVC 4.0 app still redirects and fails to find login.aspx

Open your web.config file.

Locate the Authentication node.

Change it from:

<authentication mode="Forms" />

To:

<authentication mode="None" />

Comment here, if it doesn't help, and what is the result after changes to web.config.

As a side question - how did you add reference to the ACS namespace - via "right click -> Add STS Reference", or manually changed the web.config ?

Make sure that the required Modules are registered with the web server:

  <system.webServer>    <modules runAllManagedModulesForAllRequests="true">      <add name="ClaimsPrincipalHttpModule" type="Microsoft.IdentityModel.Web.ClaimsPrincipalHttpModule, Microsoft.IdentityModel, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" preCondition="managedHandler" />      <add name="WSFederationAuthenticationModule" type="Microsoft.IdentityModel.Web.WSFederationAuthenticationModule, Microsoft.IdentityModel, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" preCondition="managedHandler" />      <add name="SessionAuthenticationModule" type="Microsoft.IdentityModel.Web.SessionAuthenticationModule, Microsoft.IdentityModel, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" preCondition="managedHandler" />    </modules>   ... </system.webServer>

Also make sure that you have microsoft.identityModel section, and you have federatedAuthentcation node within it:

<federatedAuthentication>        <wsFederation passiveRedirectEnabled="true" issuer="https://[your_namespace].accesscontrol.windows.net/v2/wsfederation"                       realm="http://127.0.0.1:81/" requireHttps="false" />        <cookieHandler requireSsl="false" /></federatedAuthentication>

Where "realm" shall be valid URL for Realying Party Application. And requireHttps="false" is to simplify development process.

When you debug it locally, make sure you are running the Cloud project (which uses IIS), and not the Web Project (which will use Cassini / webdevserver, which does not understand the system.webServer section!)