I repeated the script run overnight, after killing a few more background programs and services.
The console's last two entries were:
2017-02-02 07:55:47 Script finished
2017-02-02 07:55:47 Error: bad allocation
Looking around, I see I do have a Dense Cloud.
I also have two chunks: One of 4110 cameras. One of 0 cameras, which I deleted.
In my 2 x 5 split, it looked like one rectangle would have nothing in it. Could that have been the cause of the "bad allocation" message?
Then I generated a Mesh.
Then I generated an Orthophoto with default resolution (1.04m). This was successful.
The images are closer to 0.2m resolution, so now I'm trying that.
UPDATE: "Can't write pyramid data".
Trying again now with much more free HDD space.