Yes, I see the cable might be wrong.
USB is limited to 5 meters cable lengths without amplification (non active USB cable), however the longer the cable is the higher the risk for noise problems will be,
mostly because the impedance of the ground wire will become higher and so the 2 ground points (PC and the external board) will be worse pinned together.
We recommend no more than 2meters of USB cable.
Anyways, that error message is sure not caused by a wrong USB cable, except if the cable was not working at all (cut cable) and so then the UC100 was never recognised by the computer, then the USB drivers could not be copied by Windows and that again causes a Mach3 defective plugin message.
Unfortunately Mach3 does not show details about why it can't run the plugin, it only shows this general message whatever problem there is with the plugin.
And I agree about the importance of customer support, I'm working for more then 10 years in customer support at CNCdrive.
And thank you for the info about the Amazon seller, I don't know yet who is selling at Amazon (I've never used Amazon services, because we are not in the USA and there is no Amazon in the EU), but will try to look it up and ask why they did not give support.