Since last thursday I've been trying to figure out my problem alone but failed.
I have the last WP e commerce version installed, and it's creating conflicts with the blog pages :
when you go on the category Presse of my blog : http://ricochet-livres-jeunesse.fr/category/presse/
and you click on the pagination link "Suivant" you will reach a page which lists some products of the WP e commerce plugin.
I also desactivated the WP e commerce plugin, and saw that indeed when I am on a blog category and click on "Suivant" (Next) the pagination works...
I know that the problem comes from the WP ecommerce plugin, but I don't know how and where to fix this.
Thanks for your help !