Thanks for your message @SamO
That thread is addressing a different issue. The person who opened the topic only set up a proxy through a path and pages on that path required resources on sibling paths. They needed to add additional rules for those uncovered paths.
I’ve also reviewed the guide at [Support Guide] Making redirects work for you - troubleshooting and debugging
But my issue is different: the proxy mainly works. Only it won’t work when the communication needs to be transactional. i.e. for security reasons, the server needs to identify a user correctly, such as when logging in. It probably is due to some missed custom header configuration that I’ve not been able to unravel myself.