There's a bug in the Qu-16 firmware. It's not MSP, because it can be triggered from the iPad, too, but you might want to work around it.
Qu-16 only has 24 USB channels accessible from its Setup-I/O Patch-USB Audio menu. iPad and MSP will let you assign up to USB32. The bug happens when you assign something to USB25. The mixer surface crashes, presumably in response to the meaningless instruction. Audio still passes, but there is no display or control from the surface or the tablet. Neither tablet notices the missing connection. Reboot required. On reboot, iPad hollers about the dropped connection, but MSP never does.
Qu-32 obviously doesn't have this problem, since all available instructions are valid. I expect the same is true of your Qu-24.
My wishlist requests is limit the Qu-16 USB routing grid to to the 24 channels the Qu can natively assign.