A few months ago I had a problem where my newly created security group "Team Lead" couldn't see the small Group Tree viewer on the side when they were in the Group Viewer area.  I got some wonderful help and changed the API settings to allow it to show.  

I am having a similar problem now with my Registrations.  The Category Tree View doesn't show.  I followed the same steps as before (System > Security  > REST Controllers > Groups   api/Groups/GetChildren...). I replaced 'Groups' with 'Categories' and selected the same api/Categories/GetChildren... and added my Team Lead to allow to view.  However, I am still getting the same error (well not really an error message, just the word "Error" in a red box where the Tree viewer should be.  

I've double checked the security settings from the top down on the Pages under the CMS Configuration tab to make sure that Team Lead has view access and it appears they do.