I have a reply from the user (she is a production manager at a publishing company). Filenames starting with numbers are not a problem, even to Apple (to iPad and iTunes Producer). She said that they have 102 titles currently active at Apple and 136 that have been submitted and are awaiting acceptance, and all were created with Atlantis Word Processor. They always submit Atlantis EPUBs "as is", without "tweaking" them. That "faulty" eBook is the only file that keeps rejecting. By the way, that eBook contains adult material. Maybe this is why iTunes Producer does not want it to get through.
She also said that she will start the file over from scratch and rebuild it to see if that will fix it.