I realise this is an old thread, but i had a similar problem, where the output from GZM went "off", although there was no change within daslight and all the lights on the GZM were indicating as they should be! however as soon as there was a change in state, be this turning on a new scene or stopping one that was already meant to be running in daslight, everything would return to what it was meant to be doing!
After a bit of puzzling the solution turned out to be quite simple, a good quality powered hub! I assume that USB output isnt quite up to providing enough power to run the box? Running through a powered hub also has a benefit of keeping the GZM powered, so should you have a problem with your computer during a gig, the GZM will jump back to stand alone (and a carefully chosen backup state!) so that everyone isnt left in the dark whilst you sort the computer out!