If you use a recent MacBook Pro with USB-C connectors, like me you had to get a wheelbarrow of various adapters, and look for a solution to connect a good old audio-midi interface in USB-2 or Firewire.
When the connection is direct, with a single adapter, it usually goes well. But nowadays there is a very cool category of peripherals available: the dock on which everything remains connected and which can even charge the Mac, for example this one from Belkin which is expensive but damn pleasant to use.
And there it can become more complicated because the dock goes into sleep and scrambles the automatic device recognition protocols. My own configuration looks like this:
Mac > USB-C cable (Thunderbolt-3) > Belkin Dock
Thunderbolt-3 to Thunderbolt-2 adapter > Apple Thunderbolt screen
Firewire 800 connector > Edirol FA-66 Firewire 400 interface
A real Christmas tree… In practice, it works fine, without any extra latency, but with an extended delay while initializing CoreMidi. Each adapter must create a “tunnel” to pass the requested protocol, and the stack ending in Firewire is particularly dusty at startup.
When the first Midi application is opened, it can have a little too short time limit for this initialization, where others are more tolerant and ready to wait long enough.
Currently, with High Sierra and Logic 10.4.1, the launch of Logic fails on this configuration because Logic does not wait long enough for the whole chain to self-configure. Same with GarageBand. It doesn’t crash, but the interface is ignored and the application displays a not terribly useful error dialog.
On the other hand, no problem with StudioOne (a serious alternative to Logic, which I really like and will talk about on occasion), nor with this little gem of a Midi monitor that I warmly recommend.
Magic: just install this monitor and open it first to initialize CoreMidi, then forget it (or use it, it can become extremely convenient). If CoreMidi is already warmed, there is no more error when Logic or GarageBand is opened.