And, finally, a solution to this problem.
At a few places in the previous version of my application, I wrote
set prefsFolder to (path to preferences folder from user domain)
This seems to be the cause of the problem on my users Mac.
This line does not generate the error directly, but causes prefsFolder to be
a faulty alias which errors when next used.
While trying to test the problem, I used
set prefsFolder to (path to preferences)
which also caused a problem on my users Mac.
The correct usage is
set prefsFolder to (path to preferences from user domain)
If you add "folder" or eliminate "from user domain", it may work on your Mac,
but there is a Mac out there somewhere that will have a problem.
The StandardAdditions dictionary shows
[from system domain | local domain | network domain | user domain | Classic domain] : where to look for the indicated folder
indicating that this is optional. My experience indicates otherwise. This should be required.
Using "preferences folder" instead of "preferences" should generate a compile time error, but it does not, and, again, there's a Mac somewhere that will choke on this.
Should I pass this email along to Apple as a bug report?