There is currently a bug in the Apple Core Audio driver that will prevent the Scarlett 18i20 from streaming audio with the device set to 96kHz. This happens because the channel format doesn't change for the higher sample rate.
To resolve this please go to Applications > Utilities > Audio/Midi Setup. In there please select the 18i20 and change the channel format from "20ch-24 bit integer" to "16ch-24 bit integer" on both the input and output tabs. Please do this when the computer first starts, before streaming any audio or opening any audio software.
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