Sounds like a ground loop and digital noise is bleeding into your speakers.
I’ve had the same problem.
An audio engineer I knew suggested modifying the balanced cables going ot the speakers by cutting (desoldering would be less permanent) the GROUND wire (not the two signal cables). Problem instantly solved. It’s effectively a ground lift - which the isolation box you mention may well do.
It’s a simple enough job to desolder and resolder it if doesn’t work. Perhaps give that a try.
The balanced cables alone may resolve the issue.
If not then try the ground disconnect approach.
Not sure what the USB isolation box is doing, but I’d be inclined to save the cash and just disconnect the ground on the balanced cable first if the normal balanced doesn’t resolve it. You can do just one cable and speaker to test once you’ve got you balanced cables. Been working for me for years now.
I do not see a issue in this case…
The German forum is not very active and sometimes it takes days to get an answer.
So if it’s urgent, triyng to get help in the international forum makes totally sense.