Sharepoint isn't accepting new Credentials initially when switching users

I have a standard website (one webapplication and one site collection) with some custom pages and webparts.

The issue I'm having is that when I try to switch users, using the "Sign In As a Different User" and entering new credentials (even for another site collection admin account), IE tries the account three times, and then it presents a 401 Access Denied screen.

After that, if I erase all the stuff of access denied page from the browser's url, I'm logged as the new account I just had entered and was not accepted.

After researching for a while on google, I found a KB ( http://support.microsoft.com/kb/970814 ) that might relate, but just tested here and it didn't work at all.

The modified method suggested by the KB is the following:

function LoginAsAnother(url, bUseSource) 
{
    document.cookie="loginAsDifferentAttemptCount=0";

    if (bUseSource=="1")
    {
         GoToPage(url);
    }
    else
    {
         //var ch=url.indexOf("?") >=0 ? "&" : "?";
         //url+=ch+"Source="+escapeProperly(window.location.href);
         //STSNavigate(url);
         document.execCommand("ClearAuthenticationCache"); 
    } 
}
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But after making this change, it no longer asks for a new credential.

Any ideas?