Hi Gregoor,
See my reply to BasseemS "FN key not working." The keyboard backlights on my three month old Satellite P755-S5381 suddenly stopped working. I determined that my Fn key was also not working at the same time.
I uninstalled/re-installed Toshiba's Value Added Package. Now my keyboard backlights are on all of the time, and my screen display is on bright all of the time, but my Fn key is still not working. Maybe this will help.
Deb