• Does this happen to just one particular book? Have you tried uploading another and still get this error?
• If you're uploading a PDF file, ensure sure it has no encryption on it. Our system will automatically encrypt your document with military grade algorithms immediately following upload.
• Adobe's DRM scheme is a little stringent on the formatting of ePub files. Even documents that go through epubcheck can get stuck during the encryption phase though that is a very rare case. We'll dig through our server logs to see if we can produce some actionable items for you to get these up and running.
• In the meantime, please run the file through the epubcheck tool found at https://code.google.com/p/epubcheck/ Its output could also be helpful in identifying the problem.