*URGENT* You don't have permission to access /wp-admin/nav-menus.php on this server.

Hi,

I am running a WP multisite network with thousands of sites and I activated the MD plugin yesterday.
I just noticed that for all the sites that have been signed up since this time, the the dasboard 'appearance / menus' menu is not yet accessible.

The page shows a 403 error with message:

Forbidden

You don't have permission to access /wp-admin/nav-menus.php on this server.

Do you have any idea of what is happening ?

thanks for help!