Nicolas, thanks for the idea. That was exactly right.
In my custom.el file, the org-agenda-files turned out to be set with a static value of some files.
I don't know how it got there, but now I have deleted the entry in the custom.el file and everything is back in order.
Many thanks for solving this problem; it feels great to be able to again use my system for monitoring dates for different set terms I closely have to follow. It would have been a mess should I had to reconstruct those dates some other way.