Follow-up: I deleted the password that was saved in the
layout and entered my current password. The most likely sequence is
this one:
1. My password expired on the server.
2. I changed the password during a remote session connection,
but did not connect the dots, so I didn't change the password saved
to the WebHelp Pro layout.
3. Several weeks went by before I tested the publishing
feature again, so I forgot what I had done in step 2.
I have a strong suspicion that I tried changing the layout
password after I first discovered this issue, but it's possible
that (a) I entered the wrong password or (b) my memory is faulty.
Regardless, the project publishes correctly now, and the mystery
has been removed (somewhat).