Thanks for the feedback. My hunch is that there was some kind of problem in one of the database tables. I upgraded the system from 3.2.1 to 3.3.1, and the problem still existed. I then updated the database, and it took a while to add unique codes to each user_user.
So I don’t think it was in the phpList code, that hasn’t changed for years. My user said they didn’t do anything (I suspect that they imported a list or something like that).
I looked in the database in the config table and didn’t see anything that looked odd, so now that it’s working, I’m going to not look back.
Thanks, I appreciate the input.