Got it!!!
It was ShareData, even though there were not ShareData statements in the PBL with the relevant objects... When I rebuilt the datawindow, modifying one that had previously worked, this time I added a couple of columns that were missing... No particular reason. It just seemed cleaner. THEN when I ran the app, I got a ShareData error!!!!! It's an old, complex application, and the objects in the PBL I searched are used on or accessed by tabpages in a complex window. I guess the Share statement is higher up in the hierarchy. No time to track that down. Anyhoo, I said that I had made the same changes in the other similar datawindow. I now understand that I made those changes (reversing the order of the columns) in the SQL, but when it didn't solve my problem of trying to reverse the display columns, I must have exited without saving. I didn't get an error because both columns were strings. When I added two more columns, I got the error, because the datatypes didn't match.
So relieved to have the solution!