Paul,
I realized that Python 2.x is not installed on the system anymore. I was a bit shocked to see this, since it was the system’s default. A quick google search provided me with the information that Apple indeed removed Python 2.7.x with one of their latest OS updates (that I installed).
Probably this is the reason why AFNI now fails everytime it needs Python (even though I ran my terminal/shell using Python 3.x before). But who knows if AFNI "internally" still used Python 2.7x for some AFNI programs.
Maybe this is the problem; not just for me, but for the other users too. This seems reasonable because the other topic with the problem we are talking about appeared in the time Python 2.7 was removed from Mac OS.
I don't know how to fix it though, i.e., telling AFNI to use the currently installed version of Python.