O.K. I'm back with my experience.
The problem was that I was trying to downgrade the software version.
In order to get the appropriate version I started Nokia Suite and clicked on 'reinstall' option. This downloaded the latest version of the firmware in a folder called Ovi inside my user's folder.

This is the file I used to reflash the device with flasher under linux, becuase Nokia Suite was complaining about problems doing backup of messages/applications etc.
Now, back to the duty of setting everything as it was in the first place ....

If someone needs this file, I can provide it

Regards.