simple FormsAuthentication in a secure environment

[Craig]

I'm doing a very simple call like this:
if (MyUser.MyCustomAuthenticate(txtUsername.Text, txtPassword.Text))
{
//set a cookie
HttpCookie myCookie= new HttpCookie("cookie_name");
myCookie.Value = txtUsername.Text;
myCookie.Expires = DateTime.Now.AddDays(7);
Response.Cookies.Add(myCookie);

//forms authentication
FormsAuthentication.RedirectFromLoginPage( txtUsername.Text,
chkRemember.Checked);
}

This works fine in multiple environments, but now I'm attempting to run this
code on a server with very limited security rights. I believe that the
RedirectFromLoginPage is not setting the security ticket and cookie.
Assuming a very strict environment, what should I look for in order to give
FormsAuthentication.RedirectFromLoginPage to do what it needs to do?