I’d be leaning towards excluding the software from being the problem. Did you do your “I have done” list on both?
What you wrote about Serato would have be thinking about latency first, but you say you fiddled with it. Not sure what fiddling means, but let’s say it’s not latency. Another thing that spring to mind would be USB (are you using a hub and if so, is it properly powered?). Another USB problem could be the cable (assuming that since the sound is going, it would be the cable running to the Z1). Did you try swapping cables out?
Have you tried it with just the Z1 hooked up?
Can you get/borrow/rent another Z1/F1 to exclude that it’s a hardware problem?
Obviously I don’t have a direct answer as it’s not something I have come across before. Hopefully my suggestions give you a few other things to try.