ZAP HUD cannot Active Scan POST requests

[mavensecurity]

Problem

Active Scan for POST via HUD either does nothing (on Windows) or throws error (on Linux).

Steps

Sample target: https://php.testsparker.com/auth/login.php
Browser is Firefox launched via HUD in Zap.
Using ZAP History to find the login POST and click Active Scan.

Result for Windows

Nothing happens. Viewing ZAP GUI shows no active scan has been started.

Result for Linux

Modal alert in browser that says, "API Error - Error: URL Not Found in the Scan Tree".
I did not see anything significant in zap.log nor console log, even when DEBUG mode enabled for logging.