Engine Yard staff here.
Looking at your error, you are trying to call an each statement on Nil somewhere. Probably in your custom rake task. We can't really debug it without seeing more of your code.
Also I can recommend plain Cron tasks as a better way to run regular tasks on Engine Yard instead of using clockwork.
- At the bottom of your environment click on Crontabs (picture http://d.pr/i/3Yeo )
- Add a job by giving a job name, a command e.g. for running a rake task:
cd /data/APP_NAME/current && RAILS_ENV=production /usr/local/bin/bundle exec rake task_name
- Give it a user and time interval and press create.
- Back on your environment press Apply.
If you would like a more ruby method of setting up cron tasks in rails, there is a nice gem called Whenever which allows you to write cron tasks in ruby but they get written to the crontab. A Google for whenever on Engine Yard returns some posts on how other people have went about doing it but for doing something as simple as you showed, my above instructions would probably be the easiest.