Old post, but for those who stumbled upon this problem, the answer is to add
f.input :name => 'authenticity_token', :type => :hidden, :value => form_authenticity_token.to_s
to form. This returns an authentication token on ActiveAdmin so that it can confirm that the fake did not succeed. Your session was terminated and you were returned to the login screen because ActiveAdmin thought you were trying to fake the view.
Your form should look like this:
form do |f| f.input :name => 'authenticity_token', :type => :hidden, :value => form_authenticity_token.to_s f.input :type => :text f.input :type => :submit end
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