Thanks for all your analysis
Seems pretty logical.
However, bad news for me: I have the same problems with calibre portable, so the problem seems remaining on the book, not on calibre.
What should I do then? I thought about restoring PW to factory settings. But, if that doesn't fix the problem, my PW would be unable to add my books again =/
Any other ideas what would be the origin of the problem? Or a way to fix it?