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.