This was due to this specific customer's settings and network setup, and not due to an add-on defect.
Specifically, related to proxies, which the add-on blocks by default, as part of the intended design.
Didn't work on 1.5 for me. No response from add-on author here or on their own forum. Had to raise dispute with PayPal to get a refund, which still didn't elicit a response from anyone. Appears to be unsupported therefore?
Specifically, related to proxies, which the add-on blocks by default, as part of the intended design.