I got this error on version 1.0 and 1.1 of my latest app I submitted. They helpfully attached the cause of the error which happened to be two images, which even though it was copyright free, were somehow causing this error. They were in my launch screen and in app icon. So I uploaded my 1.1 version removing the images from my Xcode asset folder, app icon, and launch screen, and also in the iPhone screenshots on the submit page. Again I got the same error, and while waiting for the build to be distributed, I realised I hadn’t removed the trouble-causing image from my iPad screenshots. Once I removed it from there, it was a breeze. So look at the attachments they send, if it's images that's causing the error, remove them from all places where you have uploaded. Hope it helps.
