Damir So, just to explain further. I am using an iPad Pro (w/latest ios), connecting to my Ui24R via usb c to b to play backing tracks from ST3. Everything connects up fine and I can play my tracks until the sound goes dead (this could be after 20 seconds or 45 minutes - seemingly random). The only way to get sound again is to cycle the mixer off/on.
I just realized yesterday that this occurs with ST3 and any other music software on the iPad. I tried different cables and there was no success. I took one of the same cables, connected it between my Windows PC and the mixer and I haven't had a drop since. I played a couple hours of music this morning - rock solid. With a reasonable amount of certainty, the problem is the compatibility between the iPad and mixer. I am going to try a MacBook which would be fine for me to use on stage for our tracks. I'll report back what I find.