Here is a quick update.
Our system admin gave me a new VM. I did a fresh installation of Portal for ArcGIS and Web Adapter for Portal according to ESRI document. No problem so far. I am able to add map services to my map. But.... once I set up security to use Integrated Windows Authentication, I cannot add map services to a map. I receives the same error message as before - "The layer, layer_name, is not responding. Still trying ....".
An ESRI web consultant is visiting my company later this week for a different project. I will try to get some answers from him. Thanks for all your feedback.
Luke
Have there been any updates from Esri on this? In todays "the sky is falling" security environment, unchecking the "Require SSL" is not an option, yet we have a requirement to load non-SSL map services with our own SSL services. Likewise, telling users how to circumvent the "mixed content" warnings is not a good idea either. Certainly there must be some sort of proxy configuration that can act as a work-around? Can't imagine Esri put Portal 10.3.1 into production knowing this limitation.