uninitialized constant > ActionCable::Server::Configuration::ApplicationCable

The error is related to not having the correct ActionCable files in your project. You need app/channels/application_cable/channel.rb that looks like this

module ApplicationCable
  class Channel < ActionCable::Channel::Base
  end
end

and app/channels/application_cable/connection.rb

module ApplicationCable
  class Connection < ActionCable::Connection::Base
  end
end

If you upgraded from previous version (when actioncable was not in the bundle), check if config/application.rb contains

require "action_cable/engine"

I had the same problem when trying to use ActionCable with Rails 4.2.5 and the solution was to add the following to config/application.rb:

config.autoload_paths += %W(#{config.root}/app/channels)

This is with the ActionCable gem:

gem 'actioncable', github: 'rails/actioncable', branch: 'archive'