Rails 2.0 ActionMailer breaks my redmine render_message

As the exception message states, you now need to provide the mailer
name along with the template name. Sorry for the change in 2.0, but
this makes ActionMailer a little more consistent with ActionPack as
far as rendering goes. I believe this is the line that's causing you
issues:

body[:content_for_layout] = render(:file => method_name, :body =>
body)

method_name is no longer sufficient, you'll need the mailer name too.
Maybe "#{mailer_name}/#{method_name}" ? I don't use redmine so I
wouldn't know.