First of all thanks for the reply!
I read that threads and tried the JoseVega solution:
“Hi, I had the same issue, and I solved it adding: define(‘CONCATENATE_SCRIPTS’, false ); in my wp-config.php file just before require_once(ABSPATH . ‘wp-settings.php’);
I hope it works for you.”
It works, but only for the “add new media” button.
The “insert link” button still doesn’t work. It opens a strange gray layer (see the screenshot) with the window for the insert link options in the down-left corner, but you cannot click anywhere and you get stuck on it.