Cannot load WPMUDEV Dashboard links

Hi,
our WPMUDEV Dashboard only works for the "dashboard" link. All the others show this error message "Cannot load wpmudev-<name link>".
For instance :
WPMUDEV/plugins goes to "Cannot load wpmude-plugins"
WPMUDEV/themes goes to "Cannot load wpmude-themes"

We saw in the admin.php page that some files can't be found in the WP_PLUGIN_DIR directory :

if ( !( file_exists(WP_PLUGIN_DIR . "/$plugin_page") && is_file(WP_PLUGIN_DIR . "/$plugin_page") ) && !( file_exists(WPMU_PLUGIN_DIR . "/$plugin_page") && is_file(WPMU_PLUGIN_DIR . "/$plugin_page") ) )
wp_die(sprintf(__('Cannot load %s.'), htmlentities($plugin_page)));

And actually, we don't see any of these in :
/wp-admin/wp-content/plugins

Before we do something more, could you please help us? maybe it's just a thing to re-install or the cache to be cleared.

Best regards