Thanks, Øyvind, for this little glimmer of hope! If you manage to find out any more information, particularly which specific macOS security setting caused your colleague's problem and whether it affected PortMIDI or CoreMIDI, I'd be extremely grateful.

It's perhaps worth reiterating that Csound detects and successfully selects the IAC bus, so this doesn't appear to be a straightforward case of MIDI access being completely blocked.

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