Collector for ArcGIS (Windows) is unable to login to Portal for ArcGIS 10.3.1.
After entering a Portal for ArcGIS URL, an error message is returned indicating an invalid client ID, as shown below.
Portal for ArcGIS 10.3 and 10.3.1 secured with OAuth2 do not support Collector for ArcGIS on the Windows platform by default. This is because Collector for ArcGIS (Windows) was released after these products.
Earlier versions of Portal for ArcGIS must be updated to be used with the recently released Collector for ArcGIS on Windows.