View logs: heroku logs -n 256 or heroku logs --tail
Heroku is inherently ephemeral and distributed, which means that anything non-transactional, non-idempotent, non-atomic, requires some other piece attached in order to work. This includes HTTP sessions. In order for HTTP sessions to work, you’ll need to set up an attached storage disk or a memory cache like Redis or Memcached for it to talk to.