[Plugin: Google XML Sitemaps] Can't create Sitemap.xml

I have 2 sites :
– one in development where there is no problem with installing Google XML Sitemap without creation of a sitemap.xml file with 777 permissions
– the second in production where i can’t desperatly generate the sitemap (trying to create sitemap.xml with 777 permissions)

The difference with the 2 sites :
– For the dev one (which is OK), the ‘entry” to the site (http://julien.coillard.fr/wp-demo) is in the sub-directory of the site (same directory as wp-content, wp-admin & wp-include)
– For the prod one (Not OK) the entry to the site is the root (http://julien.coillard.fr), whereas the WordPress is in a sub-directory

http://wordpress.org/extend/plugins/google-sitemap-generator/