All..
If you are dumbfounded with the error "", then you are not alone. I just spent the last couple of days yelling at my computer and the F5 API.
If you are using "Requests: HTTP for Humans" (https://2.python-requests.org/en/master/) module/library within Python to perform http requests to the API, and you are sending JSON objects as part of the data, then they must use the syntax:
NOT:
Example:
Perform a "Config-Sync" with the API, and using "data=postData" you will receive
{u'errorStack': [], u'message': u'Found invalid JSON body in the request.', u'code': 400, u'apiError': 1}
Changing that to "json=postData", removes the error as it properly encodes the json data into the request.
Hope this helps!