Runtime error loading external preference file -- I7 6M62


Can’t find mention of this here or in Mantis. On MacOS 10.12.6 with I7 6M62, attempts to use an external file fail with a runtime error. This used to work.

To reproduce, simply test the “Alien Invasion Part 23” example from the docs:

"Alien Invasion Part 23"
A difficulty is a kind of value. The difficulties are easy, moderate, hard, and fiendish.
Understand "use [difficulty] puzzles" as selecting difficulty. Selecting difficulty is an action 
out of world, applying to one difficulty.
Carry out selecting difficulty: 
    choose row 1 in the Table of Preference Settings; 
    now challenge level entry is difficulty understood; 
    say "Puzzles will be [challenge level entry] from now on."
The File of Preferences is called "prefs".
When play begins: 
    if File of Preferences exists: 
        read File of Preferences into the Table of Preference Settings; 
        choose row 1 in the Table of Preference Settings; 
        say "(The current puzzle difficulty is set to [challenge level entry].)"
Check quitting the game: 
    write File of Preferences from the Table of Preference Settings.
Table of Preference Settings 
challenge level 
The Sewer Junction is a room.

It blows up with this runtime error: “Error handling external file (P48).”

I’ve posted a bug in Mantis:

Any ideas? Thanks in advance.