<div id="login">
  <h2><%= title "#{active_admin_application.site_title} #{t('active_admin.devise.login.title')}" %></h2>

  <% scope = Devise::Mapping.find_scope!(resource_name) %>
  <%= active_admin_form_for(resource, :as => resource_name, :url => send(:"#{scope}_session_path"), :html => { :id => "session_new" }) do |f| 
    f.inputs do
      Devise.authentication_keys.each { |key| f.input key, :input_html => {:autofocus => true}}
      f.input :password
      f.input :remember_me, :label => t('active_admin.devise.login.remember_me'), :as => :boolean, :if =>  false  #devise_mapping.rememberable? }
    end
    f.actions do
      f.action :submit, :label => t('active_admin.devise.login.submit'), :button_html => { :value => "Login" }
    end
  end
  %>

  <%= render :partial => "active_admin/devise/shared/links" %>
</div>

<div id="login">
  <h2><%= title "#{active_admin_application.site_title} #{t('active_admin.devise.login.title')}" %></h2>

  <% scope = Devise::Mapping.find_scope!(resource_name) %>
  <%= active_admin_form_for(resource, :as => resource_name, :url => send(:"#{scope}_session_path"), :html => { :id => "session_new" }) do |f| 
    f.inputs do
      resource.class.authentication_keys.each { |key| f.input key, :input_html => {:autofocus => true}}
      f.input :password
      f.input :remember_me, :label => t('active_admin.devise.login.remember_me'), :as => :boolean, :if =>  false  #devise_mapping.rememberable? }
    end
    f.actions do
      f.action :submit, :label => t('active_admin.devise.login.submit'), :button_html => { :value => "Login" }
    end
  end
  %>

  <%= render :partial => "active_admin/devise/shared/links" %>
</div>

Use authentication_keys from the scoped resource not global Devise.
Use authentication_keys from the scoped resource not global Devise.