Oops. The persistent error did recur, but I found another error in the source
file which might have been mis-identified. More about that later.

However, now about forty new errors appeared, all to the effect:
"Error: "Previous cache file not found" (-1) at link"

On checking, I see that there are files, present in the target website, that
have not been captured in the HTTrack copy on my hard drive. They are nearly
all images (.bmp or .jpg) which have similar filenames to html files in the
same folder as the missing file, for example: "Data.jpg" and "Data.html" in
the same folder. Some of the un-detected files are simply slow to load and
aren't interfered with by a similarly named html file.

These files haven't been erased/renamed except during the ISP's transfer of
the target website, which I thought had been fully propagated several days
ago. All the ISP did was move them from IP address A to IP address B. 

The present set of error messages apply to a narrow range of folders, all
nested seven levels deep into the target website's directory structure, the
most depth of any on that website.