I have narrowed it down to the Display -> Anchored Small Frame setting. I have left all other settings at default.
If I uncheck the Display -> ‘Anchored Small Frame’ checkbox, the error is encountered repeatedly.
I occasionally get the crash when just accepting the “Fighting Fire with… Water” (100%) quest used in the testing above.
Possibly related or a by-product of the ‘Anchored Small Frame’ issue I reported in the other thread?