Yamaha P60 - C and F# keys stopped working from Middle C upward

Dear all,

I have a Yamaha P60, bought 8 years ago. Last month, 9 keys stopped working: From the Middle C, all C and F# keys do not operate any more. There is no MIDI signal on these keys either. I have opened the Piano and cleaned the keys/contacts/rubbers, still doesn't work. I've also contacted Yamaha, but it seemed their support didn't really want to help me, the Piano starting to be old.

Anyway, I think the problem is electrical, but I dont know how to locate the issue, so I could replace the. Any help would be really appreciated.

Kind regards,
Richard