The slideshow doesn't work even with the default theme. The href tag is empty. Formerly I used jquery colorbox slideshow, and it worked fine.

`
Voir le diaporama
Voir le diaporama
Voir le diaporama
Voir le diaporama
...
`
I really doesn't know what happens. Could the apache config (main config or .htaccess) be involved in this issue ?

Check the options of the plugin and that the images are published. And clear caches.

I purged every cache (even fx cache)
I generated new cache with cache manager I take a while.

Nothing changed : I still have empty links for diaporama.

For instance : http://www.benoitvarret.fr/nature-sauvage/

Did you set your full images to protected? Set them to unprotected. Also review your error logs (server and zp). It is certainly working for me:
http://zenphoto.maltem.de/album1/

I set them to unprotected and indeed, it worked !
thanks !

Previously, it used to work with protected pictures, but it make more sense the way it works now.