I get this error trying to run a local rails server for an app upgraded to Rails 5.2 beta2:
gems/actionpack-5.2.0.beta2/lib/action_dispatch/middleware/stack.rb:108:in `assert_index': No such middleware to insert before: "Warden::Manager" (RuntimeError)
I am using Devise.
Update: Error is related to apartment gem with Devise on Rails 5.2. This line in initializers/apartment.rb
Rails.application.config.middleware.insert_before 'Warden::Manager', 'Apartment::Elevators::Subdomain'
Seems to be related. It works fine with Rails 4.10