This issue occurs with custom code that uses the WP Media modal, such as https://github.com/humanmade/Custom-Meta-Boxes or https://wordpress.org/plugins/categories-images/
The modal window does not load in IE9, except if I add define('WP_SCRIPT_DEBUG', true) so the wp admin scripts/styles are not minified.
Buttons with the “button” class don’t get the styles applied for example, so I suppose that somehow the media upload modal styles aren’t loaded either.
This happens only after version 4.0
