If you've have added the django-storages
module (to support uploading files to S3 in your django app for instance), and if like me you did not read correctly the documentation of this module, just remove this line from your settings.py
:
STATICFILES_STORAGE = 'storages.backends.s3boto3.S3Boto3Storage'
Otherwise it will cause your static assets to be looked NOT on your local machine but remotely in the S3 bucket, including admin panel CSS, and thus effectively breaking admin panel CSS.