I've taken a look at your site and have been able to gather a bit more information on the problem. This is actually related to the size of the menu being worked on. If you use the direct link to work on the footer menu you can see that the menu editor opens just fine: http://cornwallvt.com/wp-admin/nav-menus.php?action=edit&menu=16
When using the menu elements provided by Cornerstone you can set custom fields like secondary text and icons or images. This is all stored in WordPress post meta. You're not using that feature anywhere so your site front end isn't effected, but because it's possible to assign the same menu in multiple places, the menu editor needs to check if you've set any of those custom properties.
Because your menu is rather large, it's running too many queries (only when trying to edit a menu) and WordPress is aborting the request and showing a 404.
There are a few things you can do from here:
- Deactivate Cornerstone temporarily every time you want to change your menu items.
- Delete some menu items making the menu smaller. You'll need to deactivate Cornerstone temporarily to do this.
- Increase the memory limit on your server to see if this lets you work on larger menus.