Can you please test if the problem exists with 1.9.2 and/or other servers than thin (e.g. webrick or passenger)?

Thanks for the reply...

I started up the server using webrick via command prompt... bundle exec ruby script/server -p 80 -e production
Uploading the document was successful without error.

Starting the server using Thin via command prompt... C:\Ruby\Ruby193\bin\thin -c C:\ChiliProject\chiliproject-3.1.0 -p 80 -e production --prefix /myprojects start
Uploading the document was successful without error.

Starting the server using Thin via windows services (with local system account)... C:\Ruby\Ruby193\bin\thin -c C:\ChiliProject\chiliproject-3.1.0 -p 80 -e production --prefix /myprojects start
Uploading the document was successful without error.

I tried uploading files via the files section, was also successful without errors.

I have NO clue what has changed since 3 days ago and now, I don't think anything in terms of the services starting, etc... or the permissions. It has fixed itself. I will keep an eye on this and report back if it happens again.

I have the same error.

Processing DocumentsController#add_attachment (for IP at 2012-04-19 21:26:24) [POST]
  Parameters: {"commit"=>"Add", "action"=>"add_attachment", "authenticity_token"=>"qS/Ge3fpwCILdfgdgdgnzakMoT/xDtU=", "id"=>"931", "controller"=>"documents", "attachments"=>{"1"=>{"file"=>#<File:/tmp/RackMultipart20120419-15616-tmkzx6-0>, "description"=>""}}}

NoMethodError (undefined method `container' for [:unsaved, []]:Array):
  app/controllers/documents_controller.rb:88:in `add_attachment'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/rack/request_handler.rb:96:in `process_request'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/abstract_request_handler.rb:513:in `accept_and_process_next_request'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/abstract_request_handler.rb:274:in `main_loop'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/classic_rails/application_spawner.rb:321:in `start_request_handler'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/classic_rails/application_spawner.rb:99:in `spawn_application'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/utils.rb:479:in `safe_fork'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/classic_rails/application_spawner.rb:84:in `spawn_application'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/spawn_manager.rb:230:in `spawn_rails_application'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/spawn_manager.rb:132:in `spawn_application'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/spawn_manager.rb:275:in `handle_spawn_application'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/abstract_server.rb:357:in `__send__'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/abstract_server.rb:357:in `server_main_loop'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/lib/phusion_passenger/abstract_server.rb:206:in `start_synchronously'
  /home/redmine/rubystack/ruby/lib/ruby/gems/1.8/gems/passenger-3.0.11/helper-scripts/passenger-spawn-server:99

Rendering /home/redmine/rubystack-2.3.14/apps/chiliproject-3.0.0/public/500.html (500 Internal Server Error)

I stuck this code into document_controller.rb to get around the problem.
It may or may not be correct, but it seems functional (and does mail the
watchers).

  def add_attachment
    attachments = Attachment.attach_files(@document, params[:attachments])
    render_attachment_warning_if_needed(@document)

    # This begin-rescue block is used to catch the following error:
    ## NoMethodError (undefined method `container' for [:unsaved, []]:Array):
    ##  app/controllers/documents_controller.rb:88:in `add_attachment'
    #
    # For whatever reason, the attachments.first.container object doesn't exist.
    # attachments.first is an array keyed by :unsaved, which is empty.
    begin
      if attachments.present? && attachments[:files].present? && Setting.notified_events.include?('document_added')
        # TODO: refactor
        #logger.error "before first. " << attachments.first.inspect
        #logger.error "before first. " << attachments.inspect
        #logger.error "before first. " << attachments.methods.to_s
        #logger.error "before first. " << attachments[:files].first.inspect
        attachments[:files].first.container.recipients.each do |recipient|
          Mailer.deliver_attachments_added(attachments[:files], recipient)
        end
      end
    rescue Exception => exception
      logger.error exception.inspect
      logger.error "Failed to notify attachment recipients for document: " << @document.title
    end
    redirect_to :action => 'show', :id => @document
  end

The problem is that attachments has two keyed elements, :unsaved
and :files. Using .first on attachments randomly returns either
:unsaved or :files. :unsaved is an empty list in this case.
:files is a list containing a single file.

I don't know how the original code was intended to work, but
the above seems to suffice.