wp-login.php?action=postpass = 404?

I’m using a custom password form in my functions.php file, like this:

add_filter( 'the_password_form', 'custom_password_form' );
function custom_password_form() { global $post; $label = 'pwbox-'.( empty( $post->ID ) ? rand() : $post->ID ); $o = '<form class="protected-post-form" action="' . get_option('siteurl') . '/wp-login.php?action=postpass" method="post">
' . __( "<h2>Enter Password</h2>
<p>This proposal is password protected. To view it please enter your password below:</p>" ) . '
<label for="' . $label . '">' . __( "Password:" ) . ' </label><input name="post_password" id="' . $label . '" type="password" size="20" /><input type="submit" class="submit" name="Submit" value="' . esc_attr__( "Submit" ) . '" /> </form>
';
return $o;

For whatever reason, it recently started directing wp-login.php?action=postpass to the 404 page. It was working properly before, but my client said that it started acting funny over the last few days. However, he has not made any updates to the site.

WordPress 3.5.1 – I have reinstalled WP and the same problem exists.

I have disabled every plugin and the problem still exists.

I have also enabled the Twentyeleven theme and the problem still exists. Any help would be grand