Finally, I got it!
Not much to do, but very time consuming, mine took up to 1 1/2 months.
What you have to do it take off the battery and leave the player alone, absolutely not used for as long as you can.
I did it for 6 weeks and now the player works again. Nothing else worked, no format, no reset, nothing.
As I said before, I could use it as a flash memory, but not an MP3 player – it kept displaying File Sytem Err.