1. Create GB project
2. Select Live Loops at the top, and touch the "New" template
3. Click the + button to add an instrument, select Instruments
4. Swipe to the left to go to the External instruments, and touch "Audio Units"
5. Touch iSem
6. Touch the record button and record some notes
7. Touch the stop button
8. Go to the grid view by touching the grid button
9. Touch the ^ button at the bottom of the screen in the first column to play the loop
10. Touch the stop button
A message will be shown: "Audio Units extension quit" with options to Reload or Disable; both will leave the iSem AU unusable; you have to force kill GB and restart it.
I noticed the error will not always appear immediately; if you don't see the error message, repeat the playing of the loop and stopping it until you get it (until now, this happens fairly soon each time).
Am using an iPad Air 2 / 64 GB on iOS 10.2