Yeah. I have often used ground lifters (3- to 2-wire adapters) in troubleshooting, rarely if ever as a solution, and never in any system but my own where I controlled the safety ground (perhaps by rerouting to a different ground point).
In this case we are in complete agreement; I think it is noise, if not a loop, around the USB connection. Galvanic isolation should cure it. As you've said. Several times.