I just isolated the same problem. One of my team members posted this on WPML’s site before we figured out that Edit Flow was the cause: http://wpml.org/forums/topic/draft-preview-404s-on-second-language-because-url-does-not-have-language-slug/
As noted on the linked page, you can manually add the language identifier to the URL and preview will work so that it is a workaround but it gets tiresome to do repeatedly.
