I don’t want to hurry you. But I still don’t get that AU plugin loaded. I tried to change the ID to one with first character capital, as recommended here: Export to VST3 fails due to pluginId error. But that did not help. Actually, I never had problems with plugins having an ID starting with lowercase. I rather think it is the export option “AU MIDI effect plugin”. But I can’t avoid this because the point of the plugin is to issue MIDI note commands.