I am using devise and recently added active admin, which created a separate table of admin_users to keep admins.
All works fine with Active Admin when I try to log i
I couldn't make the solutions of @fringd and @coreyward to work on Rails4 (using ActiveAdmin master branch).
So, I've moved the filter methods (I have two filters: authorize_user! and check_user_status) to a new Concern, and included this created module into the Controllers which had those filters (except for the ApplicationController, which should remain clean).
Then restarted the server, and problem solved.