We've created this Forum Thread with some workarounds and advice to assist you in publishing you ebook.
Yesterday I attempted to make a few changes to a print file - I had already ordered the proof and gone through it. When I uploaded it I received the error message. I checked the forums and read about the problems on your servers - following advice I spent far too long trying to find a server it would upload to - it never did.
Today - same message keeps coming up, but I see you are telling others they have to make a pdf? That's not going to happen on my end. When will this be fixed?