cocoroco on "Edit Media not working"

What I did:
I installed WP 3.5.1 vanilla, no plugins, no themes, no nothing, just the standard installation. I logged in and went to edit the default "Hello World" post, to which I add a featured image. I then clicked "edit image".

What I expected:
The media editor interface.

What I found:

1. In chrome, a broken image is shown instead of the image, in firefox, nothing at all is shown.

   In both cases, the following request returns "-1"
   

   ../wp-admin/admin-ajax.php?action=imgedit-preview&_ajax_nonce=a64c8f6b8f&postid=5&rand=43190

   There's also a block .wp_attachment_image whith an inline style "display:none" inside of which the original image is correctly loaded in both cases.

2. If I try to scale the image, the message "Unable to create new image." is shown when I push the "scale" button

What I tried:
There are many problems pointing this problem ([1], [2], and others) but they point to problems on plugins or themes as the source of the problem, which can't possibly be my case (unless the problem is in the default theme, which will be even more severe of a bug), or they are just unsolved.
As I have no themes or plugins installed, I have no idea what to try.

My system:

• Ubuntu 12.10 (3.5.0-25-generic 64 bits)
• Apache 2.2.22
• WP permissions are 755 for directories and 544 for files, and owner/group is www-data:www-data