FATAL ERROR: OpenAComponent: result: -1,0xFFFFFFFF

This is opening an AU on an older Mac.

Does anyone know how to systematically debug this and identify the actual cause. All I can find are guesses and it’s on a customer computer so I could really do with a way of identifying the root cause! I managed to get a dtruss output from auvaltool and even that wasn’t clear!

I would say there is a good chance this is Issue with Audio Unit on 10.11