Hi Nick,
The problem is related to the Windows USB driver. I suggest two things:
1) You replace the Prolific cable with one using the FTDI chip.
2) You should take a look at the following presentation and implement some of the suggestions:
https://www.kkn.net/~n6tv/Everything%20You%20Need%20to%20Know%20About%20USB%20and%20Serial%20Interfaces%20by%20N6TV.pdfEspecially starting on pages 26-35 once you replace the cable and deinstall the drivers per the instructions in the presentation.
I am pretty sure that it will work afterwards.
As an alternative, just use the RRC's built-in USB to serial adapter instead of an external one. All you do is plug-in a USB cable and then
set COM1 to USB in the RRC's web page. Make sure that you first installed the serial drivers when installing the Microbit Setup Manager before
plugging-in the USB cable.
If you want to save money, try the second method before buying an FTDI cable, but still use the presentation's suggestions to modify the RRC's
serial settings. It most likely will also work for you.
73,
Mitch DJ0QN / K7DX