Rails 2.0 ActionMailer breaks my redmine render_message

the last line is not accepted when running rails 2.0 => ERROR
ActionView::ActionViewError (Due to changes in ActionMailer, you need to
provide the mailer_name along with the template name.
  render "user_mailer/signup"
  render :file => "user_mailer/signup"
If you are rendering a subtemplate, you must now use controller-like
partial syntax:
  render :partial => 'signup' # no mailer_name necessary)

I get the same error but I am not using redmine.

This is the function:
  def signup_notification(user)
    recipients user.email
    from "From <user@address.com>"
    subject 'Please activate your new account'
    body :url => "activation_url", :user => user

If I add:
   render "user_notifier/signup_notification"
it returns:
   can't convert Symbol into String

If I add
   render :file => "user_notifier/signup_notification"
it returns:
Couldn't find template file for user_notifier/signup_notification in "/