On mobipocket.com forums, the thread
Cybook Gen 3cannot open prc files suggests that the ePub version is still recognized as a Cybook by Windows Desktop MobiPocket Reader, with an autodetected PID, and the Desktop Reader will re-download MOBI files for you and copy them to the Cybook. On the Cybook they fail to open, but with the not very informative message: "error while opening this document: takeover.prc".
I guess it is a good thing that this isn't the final firmware. At a minimum Bookeen needs to come up with a better error message.