Here is the explanation.

Just in case somebody is in the same bad situation of having to connect a MOTU 828 Firewire Audiointerface to a new MacBook Pro: The Connection via USB-C to Thunderbolt 2 / Thunderbolt 2 to Firewire 800 / Firewire 800 to 400 works well for me.

Another excerpt from my teaching manuscript concerning Ghostnotes.