Preview doesn't show when editing image

The problem is as the title suggest.
Already tried using default theme and disabling all plugins.
My problem looks the same as this one here: http://wordpress.org/support/topic/edit-image-image-does-not-show-up?replies=10

snapshot:
http://i42.tinypic.com/1j25nk.jpg

EXCEPT it’s not about the theme I’m using.

I use firebug and the preview image’s source is:
src="<?php echo admin_url( 'admin-ajax.php', 'relative' ); ?>?action=imgedit-preview&_ajax_nonce=<?php echo $nonce; ?>&postid=<?php echo $post_id; ?>&rand=<?php echo rand(1, 99999); ?>"

But on a well-functioning site it should be:
src="/wp-admin/admin-ajax.php?action=imgedit-preview&_ajax_nonce=90556c58db&postid=1927&rand=18785"/>

I feel this might be the problem, but don’t know how to fix.
Does anyone know?
Thanks in advance for your time.
🙂