After leaving the application open for about 6 hours - there was no sound or midi afterwards.
Verified that the application is still working ok. There is some network connectivity that still worked fine for example.
Resetting the AudioDeviceManager sample rate fixed the issue and the sound was being produced again.
Does anyone have any theory about why this happened? This is the first time I’ve experienced this. JUCE has been solid on that front so far, but this is disconcerting.