I'm using the Appbuilder Beta 2. I can run startup and the java seems to start normally (listening on ports 3344, 3345, and 3346).
I enter the appbuilder address in my browser (have tried with Chrome and Firefox) and get the appbuilder splash-screen and then the "Specify the URL to your organization or Portal for ArcGIS" screen showing the address to my org's portal along with the green check.
When I click continue I'm taken to an error page on the ArcGIS Portal Directory with the following error
The strange thing is I had the appbuilder working for a few weeks prior to this and haven't to my knowledge made any changes that would impact it.
I am working on a corporate network and will make the changes suggested for the proxy tomorrow but was curious if anyone had any other ideas?