Th0mC hello, there are two settings regarding user listing:
"View user list" is a Flarum core permission, controlling who can use the API to get a list of users. It impacts user search on the website, mentions and the user directory and probably a few other things.
"View user directory" is a permission added by this extension that controls who can see the user directory page.
In order for users to use the user directory, they must be granted both permissions.
I noticed that if a visitor has access to the directory page but not to the listing, a not super friendly "something went wrong" error will be shown. I'll try improving it soon flagrow/user-directory#25