Is "message" a reserved word in Django or Python?
No. Python's reserved words do not include message and the TypeError you've described doesn't suggest a namespace collision. Look at the function's keyword arguments and make sure that message is among them. It isn't though, so maybe you meant to type msg.
I seem to get a TypeError ('message' is an invalid keyword argument for this function) every time I try adding something in the DB via the Django admin interface. The object is added but this exception is raised. Could be this linked to the fact that I have a model named "Message"?