in 7.10 the id of the password field is username_password
within suite P Improved this field’s id is user_password
if you look in modules/users/authenticate.php you will see that the authenticate process grabs the password from $_REQUEST;
if the password id is not set to what authenticate.php needs, the authenticator will not receive your password from the form and so it will treat your request as if you did not supply a password at all and not let you login