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?
The best money spent to stop all this nonsense spambots hitting my server... even though KeyCapthca stops the registration issue, they are bombing my server with the forums running upon it.
Specifically, related to proxies, which the add-on blocks by default, as part of the intended design.