Can't add custom registration fields

Hi all, just installed s2Member for a client’s site, and I’m trying to add some needed extra fields to the registration page. However, when I fill the necessary fields and click on “Create Registration Field”, nothing happens. When I click on it again, I get a JS popup complaining that the Unique Field ID already exists, even though it doesn’t show on the list.

On further inspection, Chrome’s console gives me this error whenever I click on the Create button:

Uncaught TypeError: Cannot read property 'stringify' of undefined ?ws_plugin__s2member_menu_pages_js=1918170835&ver=140423-878894966:2

Any ideas on what could be happening?

https://wordpress.org/plugins/s2member/