Thanks guys for the support.
I received the camera yesterday and I did some tests at night. Now everything is as it shod be. The only drown back was that they did not changed the camera but the PCB + sensor structure and the body is the same. Nothing wrong here but as nonprofessionals as they are, I found A LOT of dust inside which was quite difficult to clean (but in the end, that was also resolved).
To be honest I've tried 11 months to convince them to resolve the problem and was not acknowledged by Samsung Romania. In the end I recorded the conversation with the boss there which started to yell at me in the middle of our conversation and in the end I remained him of that; I god a mail next day to send the camera in warranty
and the rest I told you already.
On the papers, I received the same funny thing as usual: "we did not find any problem with the camera, the descried problem is due to sample variation, and to avoid it you should use the camera between ISO 100 and 400" LOL
I'm very satisfied with my camera now, but this will be my last Samsung product ever bought from Romania (I hate passing through the same circumstanced with the warranty, if there is a problem with the product).
Best regards to all of you