I was dusting off an old Django project and everything was working except the admin site:
Looks like your browser isn’t configured to accept cookies. Please enable cookies, reload this page, and try again.
The culprit was SESSION_COOKIE_DOMAIN = "onyxfoundation.org" being set.
The project wasn’t loading into Development mode (it’s triggered by hostname) so it set the cookie domain to what I require in production. Once remedied, everything worked as expected.