Xali on "500 Internal Server Error with new sites in multisite"

Hi everybody,
Hope I can find some help here...
I have installed wordpress in multisite and everything works well with the main website http://www.secretsdumondesauvage.fr and the secondary website namibie.secretsdumondesauvage.fr, mapped as http://www.namibie.name.
But now I'm trying to install a new site. It works well as a subdomain (madikwe.secretsdumondesauvage.fr) but when I map it as http://www.madikwe.fr I get a 500 Internal Server Error.
Anyone could point me to what's wrong?
Thanks in advance,
Magali