Suddenly my admin pages won’t load in Padrino (Template Engine not found)

I’ve never had an issues with the admin pages in my app before, but after a reboot of my machine I’m getting this error:

RuntimeError at /admin/sessions/new
Template engine not found: /sessions/new

With this abbreviated backtrace:

/Users/jeremysmith/code/robusto_server/admin/controllers/sessions.rb in block (2 levels) in <top (required)>   render "/sessions/new", nil, :layout => false
/Users/jeremysmith/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/webrick/httpserver.rb in service
si.service(req, res)

Everything in app.rb is the same as it was when this used to work:

set :login_page, "/admin/sessions/new"
enable :sessions
disable :store_location
access_control.roles_for :any do |role|
  role.protect "/"
  role.allow "/sessions"
access_control.roles_for :admin do |role|
  role.project_module :stat_definitions, "/stat_definitions"
  role.project_module :accounts, "/accounts"

I ran into this same issue using modular Sinatra pattern application and slim for my templating engine. I couldn’t render templates or partials. Here is how I fixed it:

# Gemfile
gem 'padrino-core'
gem 'padrino-helpers'
# app.rb
require 'padrino-core/application/rendering'
require 'padrino-helpers'

class App < Sinatra::Base
  register Padrino::Rendering
  register Padrino::Helpers

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