Solution
Thanks to this gist form Steven Harman, I got it working. devise_mail_helpers.rb
module Features
module MailHelpers
As others have noted: the reason is that the view which generates the mail which includes the reset password link needs to be changed.
I saw this error because I was still using the devise-i18n-views gem, which generates the old link. Removing that gem and relying on the views which are now a part of the devise-i18n gem solved the problem for me.