Ninja forms submissions not loading

I am using Ninja forms on a website. I gave one user the access to view the submissions. He can see the menu item. But when trying to load the submissions from a form. All he sees is a spinning wheel and the console shows:

Error:{ "code": "rest_forbidden", "message": "You have no permission to do this", "data": {"status": 403}}

I used the following code in the functions.php to manage the capabilities (and the plugin user role editor):

add_filter( 'ninja_forms_admin_parent_menu_capabilities',   'nf_subs_capabilities' ); 
add_filter( 'ninja_forms_admin_all_forms_capabilities',     'nf_subs_capabilities' ); 
add_filter( 'ninja_forms_admin_submissions_capabilities',   'nf_subs_capabilities' ); 

function nf_subs_capabilities( $cap ) {
    return 'nf_submissions'; // EDIT: User Capability
}

What did I miss?