This seems to (crossing fingers…) do the trick.
as a temporary solutions until i find the time to move the entire system to my living room for more appropriate fix. using a dremel i striped the plastic off the short cable that connects to smoothie and grounded the shield to earth. this means that now the USB extension cable is grounded all the way from PC to Laser and is not floating anymore.
On the left is my 10m active usb extension cable, on the right is the now grounded shielded usb cord connected to smoothie.
