plus symbol simply opens widget – doesn't work

Here’s a fix.

add this to your functions.php. Minor CSS fixes it.

function custom_admin_css() {
   echo '<style type="text/css">
.widget-top a.widget-action, .widget-top a.widget-action:hover {
    padding: 1px;
    display: inline-block;
}         </style>';
}

add_action('admin_head', 'custom_admin_css');