Hi Andrew, and thank you for your message,
I have indeed edited the header.php but after having the issue (to add the facebook fanbox xhtml condition).
So I don't think it comes from a thing I've done by my self, manually at least.
FYI, the issue is the same on the Audihome website, whose Header.php hasn't been edited at all (once again, manually at least).
But, I'll use your advice to study more carefully the problem, I'll post here if I find a solution.
Thanks again.
Alex