Analog clock sync becomes erratic while USB B cable in use

help please!

clocking S2400 via analog in (clock speed set to 16ths) via Ableton using an ERM multiclock which outputs analog clock, everything works solid. I change bpm on Ableton, S2400 syncs to same tempo.

but as soon as I plug a usb cable out from S2400 USB B port to enable audio over usb on my Mac, clock triples in speed and not an exact multiplication. goes from 100 bpm to 410 bpm.

I had this exact set up working a few days ago, clock speed was set to 16ths on the Isla and was locking no problem and using usb over audio to record.

I’ve since tried every clock setting but nothing syncs correctly when usb lead is plugged in. I’ve unchecked all the boxes on the USB sync settings, but still no joy.

if I use the ERM to sync via midi with USB cable plugged in, Isla again, syncs no problem.

any help gratefully appreciated.