I can't tell you what's going on, but I can say with certainty that modifying backups isn't it.
I thought I'd do a bit of detective work using CleanApp since, when an application is selected for deletion, CleanApp will list a large number of items associated with the application.
I didn't intend to delete Excel but I thought the expected CleanApp list of associated items might include a clue to what was causing the problem. i.e. Excel continues demanding passwords on backups of a file that no longer needs one.
However, when I selected Excel in the CleanApp list of applications, I got this dialogue:
CleanApp hasn't found any related filesThere are no entries for this file within the CleanCommunity.
CleanApp's Logging service doesn't know the file.So, I guess we'll never know. I also guess Hercule Poirot would not be impressed with my detective work.
ryck