I’m seeing the same issue. Any changes made to the Menu Structure (adding a new item, creating a new Menu, reordering the menus, etc) do not save. I click “Save Menu”, the page reloads, and the menu is reverted to its previous settings.
I’ve tried disabling all plugins, that didn’t change anything. I also tried logging out/back in.
Note that the Create Menu functionality also does not work, it reacts the same way.