"No route" error within tests on Travis CI (passing locally though)

within a Rails 4 App I've introduced two captcha methods the app-admin can choose from to protect two ressources from bot input. We use recaptcha and simple_captcha, the latter as an option for people not wanting to use google services. The implementation of choosing a captcha system via a settings form field itself works. I added some visual feedback for the user, that he_she knows if the selected system is setup correctly app-wide. Therefore I added a route which can be queried via ajax and responds with a json containing the status of the selected captcha system as well as a status message (e.g. containing an error description), that becomes displayed in some div.

The additional feature is working, the test suite is all fine locally but strangely ~60 of our tests fail within Travis CI with the following error

Failure/Error: raise ActionController::RoutingError, "No route matches [#{env['REQUEST_METHOD']}] #{env['PATH_INFO'].inspect}"

      ActionController::RoutingError:
        No route matches [GET] "/admin/settings/captcha_system_status"
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/debug_exceptions.rb:21:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/railties-4.2.7.1/lib/rails/rack/logger.rb:38:in `call_app'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/railties-4.2.7.1/lib/rails/rack/logger.rb:20:in `block in call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.7.1/lib/active_support/tagged_logging.rb:68:in `block in tagged'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.7.1/lib/active_support/tagged_logging.rb:26:in `tagged'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.7.1/lib/active_support/tagged_logging.rb:68:in `tagged'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/railties-4.2.7.1/lib/rails/rack/logger.rb:20:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/request_store-1.3.2/lib/request_store/middleware.rb:9:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/request_id.rb:21:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/methodoverride.rb:22:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/runtime.rb:18:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/activesupport-4.2.7.1/lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/static.rb:120:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/sendfile.rb:113:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/railties-4.2.7.1/lib/rails/engine.rb:518:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/railties-4.2.7.1/lib/rails/application.rb:165:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/urlmap.rb:66:in `block in call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/urlmap.rb:50:in `each'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/urlmap.rb:50:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/capybara-2.13.0/lib/capybara/server.rb:43:in `call'
      # /home/travis/build/magdalena19/lberg-map/vendor/bundle/ruby/2.3.0/gems/rack-1.6.5/lib/rack/handler/webrick.rb:88:in `service'
      # /home/travis/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/webrick/httpserver.rb:140:in `service'
      # /home/travis/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/webrick/httpserver.rb:96:in `run'
      # /home/travis/.rvm/rubies/ruby-2.3.1/lib/ruby/2.3.0/webrick/server.rb:296:in `block in start_thread'
      # 
      #   Showing full backtrace because every line was filtered out.
      #   See docs for RSpec::Configuration#backtrace_exclusion_patterns and
      #   RSpec::Configuration#backtrace_inclusion_patterns for more information.
      # ------------------
      # --- Caused by: ---
      # Capybara::CapybaraError:
      #   Your application server raised an error - It has been raised in your test code because Capybara.raise_server_errors == true
      #   ./spec/features/users/edit_user_spec.rb:31:in `block (2 levels) in <top (required)>'

The is how the route and method are defined:

  # config/routes.rb
    namespace :admin, constraints: AdminConstraint.new do
      ...
      get '/settings/captcha_system_status', to: 'settings#captcha_system_status'                                                                                                                                                             
      ...
    end 
  end

  # admin/settings_controller.rb
  def captcha_system_status
    respond_to do |format|
      format.json { render json: check_captcha_system(captcha_system: params[:captcha_system]).to_json, status: 200 }                                                                                                                         
    end 
  end

I cannot make any sense of that message since a) the route exists b) the specs failing are in no way related to the route as they do not query any data from there

Does anyone have any idea where I could start researching on that issue? Googling around hasn't got me anywhere so far and as I said all tests pass locally...

best and thanks, Andi