500 Error on the WP-admin page

There is a 500 error showing up on the wp-admin page.
http://projet-uongozi.org/application/wp-admin.php

And the admin page with a slash added showing 404 error:
http://projet-uongozi.org/application/wp-admin.php/

The main issue is that it is not possible to login to a site from the main site of the multisite installation.