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.
 
Case closed!

/Tor



2013/10/24 Nicolas Richard <theonewiththeevillook@yahoo.fr>
Tor Eriksson <teriksson2000@gmail.com> writes:
> I have the impression that this behaviour of all files not being
> correctly added to the org-agenda-files started when I chose the
> option R[emove] file from org-agenda-files proposed by org. This
> occured when org tried to build the agenda but realised that one of
> the files named in org-agenda-files was not there. After that, it is
> my impression, the problems started.

I don't remember how the "Remove" option work, but perhaps you should
search for the string org-agenda-files in your .emacs (or init.el if you
use that), perhaps it has sneaked in a custom-set-variables declaration.

--
N.