I think it is unlikely that your comms problem will be the result of corrupted software, assuming that the 680T did connect to your computer in the past, I would look for things that have changed, e.g. did it stop working after a change in computer or a change in computer operating system / other software.
Compare the differences between the working and non working 680T, are they running the same version of software for example? , using different USB cables ?, computers?, You can eliminate some differences by using the same computer / software and USB cables
Hardware problems may include dirty USB contacts and poor or damaged solder joints between the contacts and the unit's printed circuit board, also faulty chips used on the USB interface, dirty contacts are easily fixed, soldered joints require specialist equipment and faulty chips are really replaceable by end users