I had the same problem, but I found the solution.
It is a javascript (jquery) problem. In my child theme, I had the following code
add_action('init', 'my_init_method');
my_init_method loads my js and css files:
function my_init_method() {
wp_deregister_script( 'jquery' );
wp_register_script( 'jquery', 'https://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js');
wp_enqueue_script('jquery');
...
the problem is that the funcion is called from admin panel, too.
So I changed as follows:
if(!is_admin()){
add_action('init', 'my_init_method');
}
Now it works fine. Hope to be useful.