I have a test site here (with WordPress 3.2.1)
Since I activate the multisite support, the add featured image disappear. I try to deactivate all the plugin and activate the default theme (twenty eleven with has add_theme_support ('post-thumbnail') by default) but nothing change. The post thumbnail show up normaly on the front but I can't edit them without this dialog box. Of course, I checked on the "option" tab and nothing appear too for the post thumbnail feature.
When I use WP_DEBUG, I get:
Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/patricet/public_html/wp-testsite/academie-sophro-v2/wp-includes/functions.php on line 3382
Notice: has_cap is called with an argument which is deprecated since version 2.0! The use of levels of user extensions and themes now prohibited. Use roles and capabilities in place. in / home/patricet/public_html/wp-testsite/academie-sophro-v2/wp-includes/functions.php on line 3466
In console (with Firebug) for .js I get:
The use of the property "nodeValue" of an attribute is deprecated. Use "value" instead
I can create an editor account if you need
Do you have a solution, an idea?