Rails 2.1.1 moved module Dependencies to ActiveSupport::Dependencies. That made Community Engine fail to start on Rails 2.1.1 (it’s actually Rails Engines that fails to start). You’ll get this error:

Dependencies is not a module (TypeError) 

To let Community Engine run, edit line 143 of vendor/plugins/engines/lib/engines/rails_extensions/dependencies.rb:

Change

module ::Dependencies  to 

module ActiveSupport::Dependencies