The back-end script use to have a issue that in some servers (later I noticed that it was due to CloudFlare) the returned JSON was not well formated, that was the initial problem and that was solved. Now, the error with the "no image uploaded" is quite different and I believe that it trends to happen due the flash uploader. The flash uploader doesn't work in a normal way, is different. Normal request are issued by your browser, flash request... not so quite the same. When you trigger the upload the flash is like a new computer sending the file, that is why you don't see the request in the network tool of the browser console. This means that server says OK while flash (as a browser) says WRONG.
Sadly, this is due to uploadify and it will take me ages to debug it and there is not a 100% certain of that this is the issue. After days debugging this the only thing that could be causing this problem is the negotiation between flash and the server.