Ok, I've found a fix for this.
The bug arises when the original instrument track is missing, so the trick is to try and recreate it.
So with the messed up GUI version still there, I did the following:
- Insert a new Audio track, set the Input to Kontakt 5 stereo 1
- Insert a new MIDI track, set it's output to Kontakt 5
- Select both tracks, then right-click->Make Instrument Track
Now when you open the GUI, Strummed Acoustic is back to normal.
What you can now do is Hide the new instrument track you've created, so you don't accidentally delete it.
Hope this helps!
M.