Hello,
We were actually able to solve the problem yesterday.
On the internal ADFS server, in the ADFS manager we Revoked All Proxies. We then removed the Remote Access Service from the ADFS proxy server and re-added it, and re-configured. Then, going to ADFS management on the internal server, under Service --> Endpoints, discovered that the URL Path adfs/ls/ was set to NO for Proxy Enabled. By changing to YES, the proxy began working immediately.