I had forgotten to define the location of wp-content in my wp-config.php :
define( 'WP_CONTENT_DIR', dirname(__FILE__) . '/wp-content' );
define( 'WP_CONTENT_URL', 'http://'.$_SERVER['SERVER_NAME'].'/wp-content' );
define( 'WP_PLUGIN_DIR', dirname(__FILE__) . '/wp-content/plugins' );