It could be a bad font.?
Try to create a new book with a different font and check the same are you receiving the same error.
I Had the same issue... I am not sure what made it happen. I made a new book from scratch and that worked just fine.
I have just had the same problem. I tried many approaches and had several calls with staff. I identified that one problem was with the title pages. I re-did the cover from scratch many times in many ways, sometimes successfully and sometimes not. I finally came to the conclusion that if the template used contains a text area, one must not overlap with page text. I deleted page text and unchecked the box for page text (and the same for photo text) and was able to download a PDF. I then went through my book and deleted all page text from those pages with text areas included in the template and was finally able to download to PDF and Blurb. Adobe should upgrade their error messages! James Wilton