I've traditionally having problems with mine the last couple of years... It used to work on High Sierra, but I was forced to remove my last High Sierra install known to work with the Mbox 3 Pro... Reinstalled High Sierra - I had to re-download a newer version of it (10.13.6)... and it doesn't work at all...
Maybe this thing your'e discussing here is related... Maybe a security update happened between my previous High Sierra install, and this one. I'll give it a try to see what's going on...
I'm linking the other thread I was reading last time:
https://duc.avid.com/showthread.php?p=2564123