How do you diagnose a 500 error on Heroku when there is no error message in the logs?

Try adding the rails_12factor gem to get a more robust error log ( only if you're using Heroku ).

Make sure you rake db:migrate your database on Heroku with heroku run rake db:migrate

In Heroku, I diagnose errors with LogEntries -- its far easier than the Heroku logs to diagnose errors.

What I do is load up the app, and in the LogEntries panel, go to "Live (Beta)". This shows any errors which appear, and are generally very explanatory

Just something that might help

The solution that worked for me when deploying my Django apps to Heroku is to go into the production.py file and change the code DEBUG = FALSE to DEBUG = TRUE. In this way, you should be able to see the errors that Django displays. Be aware that once you have found the error and have done the corrections, this setting must be changed back to FALSE.

In case changing to TRUE shows the app to work but the error is not visible then check your forms.py file and make sure that your models do not have any inconsistency. In my previous cases, the forms.py file has been the culprit.