Dashboard of marketPress store ERROR

I have a problem with the dashboard of all newly created marketpress stores.
The facebiz.fr/def8/wp-admin for example has an Internal Server Error.
Maybe a htaccess problem as it can cause such error easily. I have this in my htaccess file :
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
There is a DNS redirection from a different registrar for the domain to point to another host server, can this be a problem ?