I couldn’t find my other USB-USB cable so I tried the Prolific USB cable that came with the LD-5. I got the SAME RESULT as your original post reported. Oops! How frustrating.
Another issue: the exe file wouldn’t run at first. Windows 8.1 stated that it wasn’t a windows program so I deleted the exe file and downloaded it again from the LNR web site. So then my security software rejected it; deemed it a threat. I overrode the threat designation and the updatefirmware1_0_4.exe started working, but then that failure re: the COM port happened again.