… and I can answer my own question unfortunately with yes, there is a problem using the Midex with a Z170 mainboard, however it’s easily fixed.
For people in a hurry, with a free PCI/PCIe slot: plug in a USB2 card and use that for the Midex.
The longer, more technical answer: it seems that the Z170 chipset no longer supports the legacy USB modes that the Midex relies on. It look like there is a process whereby the chip in the Midex needs to load its firmware after it’s been plugged in, and this is where it’s failing, possibly as the Z170 no longer support EHCI.
Luckily In my case, there is also a USB 3.1 port on the mainboard (provided by a seperate ASMedia ASM1142 controller) and plugging the Midex into that solved all the problems. It’s incredible and somewhat ironic to think that I am able to keep such an old device alive by using a 10GBit/s USB port!