OK that's solved.

For some mysterious reason, the nokia-maps binary couldn't find any more its maps.
In Xterm, when I start nokia-maps in any directory other than /home/user/MyDocs now no maps are shown offline, only if I let it start from that directory the offline maps are used.
Apparently, the maps path is hardcoded in the /home/user/MyDocs/.qf file as relative references to the /home/user/MyDocs directory. I tried to include the full path but it wasn't working.
So I wrote a small shell script, changing the directory to /home/user/MyDocs and starting the nokia-maps binary from there. The script works, and offline data are used. From the menu, it still didn't use offline though.
So, I had to change the nokia-maps.desktop file in /usr/share/applications/hildon.
Changing the Exec= field do the shell script was not enough. I also had to hash out the X-Osso-Service=NokiaMaps line.
And now it works also from the menu.

I don't know what happened and why the nokia-maps binary doesn't find the maps directory any more. Maybe I accidentally erased some symlink or config file? Though I apt-get purged and reinstalled all the maps related package with no joy.

I suspect that an adequate symlink could be a better solution, avoiding the need for a shell script. Don't know where to put it

Maybe all this stuff could be useful to some other fellow user who can't open his maps any more...