A workaround!
The actual issue appears to be a lack of support for SNI in the ArcGIS Server proxy. Our web server was set up with multiple sites, each bound to a FQDN. On a hunch, I added a default HTTPS binding to the site that contains the Portal Web Adaptor and things began working.
At one point I did have a default binding on the site in order to configure the Web Adaptor (so I could use https://localhost/ to get at the configuration page), which is why it worked for a while and then "stopped". At the time, I didn't associate removing the default binding with the loading failures.
There is an Esri BUG-000093827 that discusses an issue with the Portal proxy and SNI support that was fixed in 10.4.1, but nothing I could find that discussed the state of SNI support in the ArcGIS Server proxy.
I'm hoping to get a resolution from Esri support that either
- Confirms this as a bug in the AGS proxy, or
- Provides some details on how to enable SNI support
For now, moving on from this issue....
TLS Extension List sent from ArcGIS Server
extensions [extension_type: elliptic_curves,extension_type: ec_point_formats,extension_type: signature_algorithms]
TLS Extension List sent from Chrome Browser
extensions [extension_type: 31354,extension_type: renegotiation_info,extension_type: server_name,extension_type: extended_master_secret,extension_type: SessionTicket_TLS,extension_type: signature_algorithms,extension_type: status_request,extension_type: 18,extension_type: application_layer_protocol_negotiation,extension_type: 30032,extension_type: ec_point_formats,extension_type: elliptic_curves,extension_type: 10794]