This patch is a thing to be use the BetterErrors in Padrino 0.11 or later. Because, BettorErrors's XHR prevent by rack-protection.

boot.rb

# This file in a config directory.
if Padrino.env == :development
  # Setup better_errors
  Padrino::Application.use BetterErrors::Middleware
  BetterErrors.application_root = PADRINO_ROOT
  BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']
  Padrino::Application.set :exclude_from_protection, '/__better_errors' # Append this
  # ...
end

rack_protection.rb

# This file in a lib directory.
if Padrino.env == :development
  module Rack
    module Protection
      class AuthenticityToken
        def accepts_with_allowed_path?(env)
          if defined?(Padrino::Application.exclude_from_protection)
            [Padrino::Application.exclude_from_protection].flatten.each do |path|
              return true if Regexp.new(path).match(env['REQUEST_PATH'])
            end
          end
          accepts_without_allowed_path?(env)
        end
        alias_method_chain :accepts?, :allowed_path
      end
    end
  end
end