After looking for a while, I realized that the new json cookies serializer on rails 4.1 breaks moped queries on devise resources.
To fix that, remove the following line on the cookies_serializer.rb initializer
Rails.application.config.action_dispatch.cookies_serializer = :json
You may want to get the old sessions_store.rb file back with content similar to:
YourApp::Application.config.session_store :cookie_store, key: '_yourapp_session'
or try the master branch of devise.
Take a look here: https://github.com/plataformatec/devise/issues/2949#issuecomment-40520236 and here: https://github.com/plataformatec/devise/pull/2882