dashboard_path refers to the rails_admin dashboard path instead of the one from the host app

application_controller.rb

class ApplicationController < ActionController::Base
  protect_from_forgery
    
  rescue_from CanCan::AccessDenied do |exception|
    if user_signed_in?
      flash[:error] = I18n.t('auth.message.access_denied')
      session[:user_return_to] = nil
      redirect_to dashboard_path
    else              
      flash[:error] = I18n.t('auth.message.admin_login_required')
      session[:user_return_to] = request.fullpath
      redirect_to new_user_session_path
    end     
  end 
end

rake_routes_output

             rails_admin        /admin                           RailsAdmin::Engine
        new_user_session GET    /users/sign_in(.:format)         devise/sessions#new
            user_session POST   /users/sign_in(.:format)         devise/sessions#create
    destroy_user_session DELETE /users/sign_out(.:format)        devise/sessions#destroy
           user_password POST   /users/password(.:format)        devise/passwords#create
       new_user_password GET    /users/password/new(.:format)    devise/passwords#new
      edit_user_password GET    /users/password/edit(.:format)   devise/passwords#edit
                         PUT    /users/password(.:format)        devise/passwords#update
cancel_user_registration GET    /users/cancel(.:format)          devise/registrations#cancel
       user_registration POST   /users(.:format)                 devise/registrations#create
   new_user_registration GET    /users/sign_up(.:format)         devise/registrations#new
  edit_user_registration GET    /users/edit(.:format)            devise/registrations#edit
                         PUT    /users(.:format)                 devise/registrations#update
                         DELETE /users(.:format)                 devise/registrations#destroy
               dashboard GET    /dashboard(.:format)             dashboard#index
                    root        /                                main#index

Routes for RailsAdmin::Engine:
    dashboard GET         /                                      rails_admin/main#dashboard
        index GET|POST    /:model_name(.:format)                 rails_admin/main#index
          new GET|POST    /:model_name/new(.:format)             rails_admin/main#new
       export GET|POST    /:model_name/export(.:format)          rails_admin/main#export
  bulk_delete POST|DELETE /:model_name/bulk_delete(.:format)     rails_admin/main#bulk_delete
history_index GET         /:model_name/history(.:format)         rails_admin/main#history_index
  bulk_action POST        /:model_name/bulk_action(.:format)     rails_admin/main#bulk_action
         show GET         /:model_name/:id(.:format)             rails_admin/main#show
         edit GET|PUT     /:model_name/:id/edit(.:format)        rails_admin/main#edit
       delete GET|DELETE  /:model_name/:id/delete(.:format)      rails_admin/main#delete
 history_show GET         /:model_name/:id/history(.:format)     rails_admin/main#history_show
  show_in_app GET         /:model_name/:id/show_in_app(.:format) rails_admin/main#show_in_app

routes.rb

Foobar::Application.routes.draw do
  devise_for :users
  
  mount RailsAdmin::Engine => '/admin', :as => 'rails_admin'

  get '/dashboard' => "dashboard#index", :as => :dashboard
  root :to => 'main#index'  
end