Proxy is no longer able to authenticate to an upstream proxy

BorderManager 3.5 added a feature called Proxy Chaining. This allows the BM server to sit behind another proxy server that requires authentication. BM will cache all objects returned for requests containing authorization headers. For subsequent requests to the came cached object, the BM proxy will request authorization from the upstream proxy and if it is authorized, it will send the object from it's own cache. This will ensure that BM will cache objects, and will also dispense them to only authorized users.

This feature, unfortunately, doesn't work after applying SP5 to BM3.8 or going to BM3.9. The upstream proxy server will send a 401 status response and the pop up with the error gets displayed at the browser.