Thanks for the replies.
I didn't found problems with the permissions. They were all set, for all
the albums, as visible for everybody, and modifiable by admin.
I did found a workaround, though. It seems the problem is the following:
I upload new photos to a "temporary" album, which is hidden. That way I can
work on how to organize the photos in sub-albums, and then I move the new
photos and possible new albums to the visible albums. I notice that if I
move a sub-album from this hidden album to a visible one, it remains
invisible. If I move a sub-album from a visible album, it remains visible.
So, annoyingly, sub-albums silently inherit the hidden attribute, and, at
least in the woody version, this cannot be unset, as the <<Edit Album>>
options only show the "Hide album", not the "Show album".
The only solution I've found is to make the temporary album visible, create
duplicate versions for the problematic sub-albums, and then move the new
versions to the desired locations. This does not make me happy because too
many albums are involved, and I didn't notice until today.
The problem only seems to appear with albums. Individual
pictures move from a hidden to a visible album remain visible, which is why
I didn't notice before.