OK I found it.
The problem was caused because WordPress uses the GD Graphic Library to manipulate images (see wp-admin/includes/image.php), which I didn’t have installed.
As far as I could find, the requirement to have mod-gd installed is not mentioned anywhere (specifically not here) , I think it would be a good idea to update the docs.
Thanks everyone!
Maybe I wasn’t clear: the request which is supposed to return the editable image:
../wp-admin/admin-ajax.php?action=imgedit-preview&_ajax_nonce=a64c8f6b8f&postid=5&rand=43190
returns a 200 OK HTTP response with the content ‘-1’, not any error response. There are NO error responses.
I even switched PHP error reporting on, as well as WP_DEBUG, WP_DEBUG_LOG, WP_DEBUG_DISPLAY and SCRIPT_DEBUG and no errors are shown on the screen, the apache logs, or wp-contents/debug.log
As I said I’m using a plain WP installation on a quite standard (x)ubuntu box.
