Network Wide Menu - Stopped working?

I had a plugin called 'Network Wide Menu' which was installed and working for a few sites on my network - it allowed the menu of the main site to be used on all sites on the network. However, as of earlier today it appears to have totally stopped working - I would normally be able to go to appearance -> Menu and create any menu, after saving it there would be a 'manage locations' tab which would appear on the menu page. From there I would select the menu from my primary site.

Any idea why this suddenly stopped working:

main site:

Let me know if there is anything I should try! Support access has been granted to the site.