I’m testing this out ..
I installed it with and used my original theme and also with your recommended theme and when I go to edit the Settings for Listing and select the page for displaying the category, then when I click save the page reloads with
Access to this resource on the server is denied!
Please let me know if it’s a local installation or via some hosting provider? It seems to be a hosting provider issue, they may block some requests for security reasons (you can ask them why this page is blocked), but this page is added via the official WordPress settings API.