large X appear on new pages in iBooks Author
I am preparing a textbook with several figures and text.
After about 40 pages the file is 60 Mb but the problem is that every new page I create has a large X covering the page.
The large X also appear in the PDF export. In the iBooks preview, though, a small "?" sign appear on top left of the page...
What does it mean?
How can I solve this?
Thanks in advance
Filippo
MacBook, OS X El Capitan (10.11.3)