Rails 6 - ActiveAdmin NameError in Admin::Users#index

Question

I'm using ActiveAdmin and I want to be able to see all the user accounts (index page), The User model (User) is created with Devise and the Admin model (AdminUser) was created with ActiveAdmin.

When I want to view the index page of users I get this error:

NameError in Admin::Users#index
uninitialised constant User::Order

This is what I see in the terminal

ActionView::Template::Error (uninitialized constant User::Order):
1: # frozen_string_literal: true
2: insert_tag renderer_for(:index)

I know that I have access to the user model because the show page for a user works.

So /admin/users/:id works but /admin/users throws the error.

I honestly have no idea what it could be, already tried to change the name of the ActiveAdmin route with ActiveAdmin.register User, as: 'Gebruikers' do but the error persists. Also found this question NameError in Admin::Users#index but the solution didn't help.