Tonight's update to ArcGIS Online introduces a new concept of "User Types". You will not be able to interact with these new user types unless you are using v1.5.2 of the Python API, also released tonight. Please install or upgrade this newest version of the Python API via:
conda install -c esri arcgis
pip install arcgis
Other new features and bug fixes associated with this release can be viewed here. This includes a MapView.take_screenshot() function, Spatially Enabled DataFrame updates, and more! Integrate this new version of the Python API into your environment, and as always let us know your feedback!