I am unable to establish a connection to any Portal in ArcGIS Pro 2.4.2. We have also tested our Dev portal and www.arcgis.com with the same result. I am able to reach the Portal site in the browser fine and the TLS settings in the internet options have TLS 1.2 checked. What is very interesting is that when I have Fiddler running while opening ArcGIS Pro, it CAN connect to the Portal and I am able to sign in.
Any help is appreciated.