If you are seeing this error while investigating Heroku logs, it might be because your write permissions have been taken away!
Whoops! How do I know that? This blog was down for a couple of days because I had exceeded the maximum number of Postgres database rows on my Heroku free account. Conveniently, Heroku now makes it easy to directly interact with Postgres data via psql.
I fixed the issue and waited 24 hours, but I was still receiving HTML 500 errors. This time the errors were limited to my django admin login page. Upon inspection of the traceback, it seemed like maybe Heroku needed to take action on their side.
2013-03-20T02:52:12+00:00 app[web.1]: DatabaseError: permission denied for relation django_session
I fired off an email to email@example.com. Sure enough, Heroku needed to (manually?) re-enable write access. It seems like this was done soon after receiving my email and escalating to a support engineer. Heroku support is top-notch, especially considering that the app running this blog is free.
I wrote this post in the chance that it may help someone doing an on-line search down the road.
Thank you Heroku!