Similar case as this one: http://www.kriesi.at/support/topic/add-image-to-slideshow-fail
Adding image to a slideshow fails, all things work as usual: spinning arrow, green checkmark appears. But no entry for the slide appears.
It worked a few days ago, then my client added some content, and now it doesn't work. I suspect an AJAX failure of some sort... Tried reinstalling WP 3.5, disabling all plugins, still doesn't work. I can still add featured images when I change the theme back to Twenty Twelve.
Would be happy to give you a login/password for you to check it out if necessary. Thanks in advance!