This is
solved.
According to the iTunes Producer manual,
Quote:
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Publication Date: Type the date (YYYY-MM-DD) of the original, physical publication. This is the date users see in the iBookstore.
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but my clients were entering the date in M-DD-YY format. Looking at the xml file generated from the failed upload,
Quote:
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<publication_date>7-01-12</publication_date>
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was incorrect. I had them enter the date exactly as the manual suggested, and all seems to be well, we've submitted the book.
Totally silly: iTunes should update that form so you have to use the proper format!