Actually this may be an issue with my 4.0 upgrade. jQuery is not defined anywhere in the admin panel and other stuff is broken as well. Reverted to 3.x and it works.
Was it working before you upgraded? (Are you running 2.4.1 — I just pushed out 2.4.1 this evening with wordpress 4.0 support).
If you’re answering yes to all this, I would suggest clearing your cache and trying again, and then enabling debug mode (Settings → Post Thumbnail Editor with the logging going to a file) and then try it again and load the log file to see what it reports.
Oh great, keep me posted.
