If you find yourself unable to capture application audio after plugging in a certain device, e.g:
"... the audio playback is not working AFTER I plugged in a Belkin Thunderbolt 3 hub. "
and / or:
"Adding on to this - the SWB Audio Capture works perfectly after I unplugged the hub."
Specify an explicit output device for iShowU to route app-audio to.
Change this in iShowU Instant's own preferences (General Tab, first option).
See below (I'm changing it to my ODAC device, yours will differ):