So I did it, and same as the first try, nothing has changed.
I start thinking to flash my device as it was out of factory ... Annoying.

Thanks for giving help proposition

-edit-
So I am trying to remember since when I had this issue. It seems to me that it appeared when I install this "Dominoes" app. I wanted to see what would happen if I removed it. I removed it from the desktop, and then weird stuff : my desktop icons were disappearing. Like lots. I switched of the phone. Seems that the phone is not starting anymore. It stays on 'Nokia' starting screen. F*ck.

-edit2-
Well. I flashed the device. It now works like a charm. Thanks for your time.