Losing all C's and F#'s is more consistent with a hardware issue than a software issue. I was looking at the service manual and having a bad connection to 2 pins in the keyboard cables could easily cause such a problem (as well as failure in or near the keyboard processing IC)

If you're really desperate and can't send it to Yamaha, a qualified repair person could open it and inspect and re-seat the cable going from the keyboard to the "MK Sub Circuit" board.

I don't really recommend this but it's what I would do, having designed, built and repaired synths for 10 years. My avatar is a CS70m that I repaired and upgraded to MIDI 😉

Thank you for your help 🙂

There is maybe someone qualified enough to have a look at it but I was wondering if it breaks the warranty if it’s opened before the Yamaha service center ?
Has someone some infos about it ?

Thanks again 🙂

Not sure if this is useful. I friend of mine had an issue with a MOXF a few years back, Octaves suddenly started moving around the keyboard, i.e. the low octave played on higher keys. At the time we were using power from a generator, not standard power; I suspected that spikes in the power may have damaged the firmware or OS. He did manage to repair it by reloading the OS and I think the firmware. You could try this, it may be a similar problem. Obviously make sure you back up before undertaking this process. The OS is fairly easy to update, I'm not sure how to do the firmware.
Good Luck