We're using the latest version of WP and Gantry.
Under Appearance > Menus, we see the following:
Categories
Sub Items...
News
Sub Items...
Articles
First Article
Second Article
Members
Third Article
Fourth Article
Fifth Article
Under Hydrogen Theme > Menu > Main Menu, we see the following:
Categories
Sub Items...
News
Sub Items...
Second Article
Fourth Article
I tried deleting the articles and members menu items and sub-menu items, and subsuquently re-creating them. I also tried disabling all plugins except for Gantry, but the error persists.