I am using WordPress 3.5 with the Twenty Twelve theme on a dev server.
The issue is : when I try to add a media / thumbnail to a post or page, the media panel that appears with ajax doesn't show any file. ( And i can't give a url for the thumbnail anymore in 3.5 ). When I add a file ( drag'n droppin' ) WP send an error "try again later", doesn't show the file on the panel, but it is in the media library and in the uploads folder.
My server isn't using mod_pagespeed, I don't have a single plugin, I use the Twenty Twelve theme and the problem exists on every browser, and chrome/firefox debug console isn't showing any error... I also tried to add "define('CONCATENATE_SCRIPTS', false );" to wp-config.php, and set WP_DEBUG & SCRIPT_DEBUG to true, but nothing changed.
I think it's probably a server thing, but wasn't able to find if WP 3.5 requires something special / new.
The same server hosts many WP on amny versions before 3.5, without this kind of trouble.
Any suggestions ?
Thanks to all of you reading this !