ArcGIS Pro 2.x loops back to the licensing portal window after signing in to Portal for ArcGIS 10.4.1 if it is configured for ArcGIS Pro licenses using some ArcGIS License Server Administrator 10.4.x machines.
ArcGIS License Manager 10.5.1 or later must be installed to manage ArcGIS Pro 2.0 Named User licenses with Portal for ArcGIS or to manage Concurrent Use licenses.
See http://pro.arcgis.com/en/pro-app/get-started/licensing-arcgis-pro.htm