The randomness of the drop-outs indicates a physical problem, not software. The latter would usually be reproducible. Especially since you took MIDI out of the equation a while ago. I don’t believe there’s a MIDI ‘ghost’ injected into all those machines, it would be gone at disconnect/reboot.
I skimmed through the other thread (now this one, Avantronica apparently just merged them now), and the problems seem to be of mixed nature, some MIDI-based, some on sudden voltage surges, i.e. any input from external sources on a different power source.
Maybe it’s a bad audio out, loose cable… or a bad IC. If it’s a software problem, I’d be surprised.
Anyway, support will help.
Good you got it to work for now and good luck!
PS. the last time I had no audio out was when I accidentally switched the first one of two subsequent neighbour machines into a static one, yes, right before a gig. Luckily, I was able to find the cause in time.