Are you measuring that 68mV on the DC or AC range? If on DC then it might (just possibly) be an artifact of a more significant AC voltage - worth a quick check.
USB 1.1 isolators are getting relatively cheap now, I had to make my own when I needed one. You need to check is that they can output enough power, a lot are limited to 1W (200mA), assuming the ST Discovery board doesn't have a mains adapter.