Yeah I have this same problem. define(‘CONCATENATE_SCRIPTS’, false); broke a whole bunch of other stuff. I’ve tried adding a new user (did not work), updating the WP core (did not work), and I have no files in my media library which are bigger than a couple of MB. I think it is a jQuery conflict of some sort, but can’t see how to fix it. I de-activated all the plugins, changed to the 2014 Theme, no dice, still happens, no matter what I try.