Why doesn’t the LCD turn on when I reboot my Dell Vostro?

I have a Dell Vostro with a strange issue, after few hours on, if I reboot, the LCD doesn’t turn on.

There is a 2 amber + 7 white blink code (LCD failure) for a while and the OS starts regularly.

If I attach an external monitor it works.

I already tried to replace the LCD cable and the power supply. They didn’t help.

Do you have any other idea? Unfortunately it is out of warranty period.