Hi Marc,
Reading the docs suggests that the cable might be to blame. Before contacting the seller about it I just wanted to check if this is the expected failure mode for the cable or would it not work at all if it were using a copycat pl2303?
If you are using Windows and you have not previously downloaded, installed and selected the Prolific V3.2.0.0 device driver, this is the problem. Nearly all chips in programming cables that identify themselves as being Prolific are unauthorized copies. The device driver automatically installed by Windows (since around 2008) is intentionally incompatible with unauthorized chip copies.
You can double check using Device Manager. It will show which device driver is selected. If it show a device driver version newer than 3.3.2.105, then it is incompatible. Both Prolific V3.3.2.102 and V3.3.2.105 will work with CHIRP, but my testing has found that it is not compatible with some factory programming software. The recommended version is V3.2.0.0 because it works with all programming software I've tried and any chip that identifies itself as being a PL2303.
This page has a link to the V3.2 0.0 device driver and a guide for how to get it selected. I keep several versions installed and switch between them at will to test for genuine Prolific USB-to-Serial chips. They are very rare. Of the many programming cables I have, only 1 has a genuine Prolific chip.
http://www.miklor.com/COM/UV_Drivers.php
Jim KC9HI