Hey Tim,
I’m using a BM Ultrastudio Express with my MBP. Been using it for over 4 years. It works like a charm.
As I understand, you plug in the analog out XLRs from the breakout cable into the Mackie. I’m currently using the analog output for monitoring. Works well.
The first thing you can try is uninstalling the BM desktop video and installing one that is 4 or 5 versions back. Hope this works.
Kannan