I know there is numerous post with this error in the new media library, with no answer yet.
Since I've upgraded to wordpress 3.5, the media library does not work. It shows no images, and when I try to upload, I have this message :
An error occurred in the upload. Please try again later.
But the image is loaded, I saw it in the media tab, and in ftp I saw the 3 images small, medium and original. The problem is there is no images shown in the media library.
Tried to desactivate all plugins, clear the cache, use two others browsers, use the twenty twelve theme : always the same problem.
My wp-config has already the line
"define('CONCATENATE_SCRIPTS', false );"
I tried to delete the htaccess, too. Same problem.
I made an exact mirror of my blog in a local server, with my custom theme and plugins, and here all is ok, the media manager works perfectly. The Php version is the same (5.3.6). Don't know exactly where is the problem. I don't use the mod pagespeed.