function load_scripts() {
wp_deregister_script('jquery');
wp_register_script('jquery', ("http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"));
wp_enqueue_script('jquery');
}
add_action('init', 'load_scripts');
I have found the problem now (for all the following readers).
In my template functions.php I’ve included the latest version of jQuery. So jQuery has been included in the admin panel too. The fact that WordPress loads jQuery in the admin panel twice must have caused the issue.
I solved this by adding the conditional tag is_admin to the function add_action
if(!is_admin()) {
add_action('init', 'load_scripts');
}