Just to be clear, all you did was edit and add this
add_filter( 'allowed_redirect_hosts' , 'my_allowed_redirect_hosts' , 10 );
function my_allowed_redirect_hosts($content){
$content[] = 'blog.example.com';
$content[] = 'codex.example.com';
// wrong: $content[] = 'https://codex.example.com';
return $content;
}
to wp-config.php?
