Hi! This exact thing has just happened to me right now!
Actually, in my case all
C, E and G# keys do the same: after a keyboard power cycle they can each send a midi ON command once, then after that they don't respond anymore. I even tried to reupload the firmware (3.0.0.5 for 'N' not 'O' serial number), without success.
As my unit is still under 2 years old it is luckily still covered by the EU customer warranty so if Arturia doesn't respond quickly to this post I will return it for a replacement.
[edit:]
After some software updates and playing a bit with the keyboard, I found out the following things:
1) that when C E and G# keys don't work, also
encoders #1 and #9 don't.
2) that sometimes it spontaneously start to work properly again. It happened first a couple times when working with the just installed Analog Lab 2. then stopped working again. Then I decided to log all incoming midi commands (with
'MIDI Monitor' for MAC OSX) and touching every single knob, key, pitch/mod strips, pads... all of a sudden, after having touched everyting, it started working properly again! and when keys C E G# were pressed for the first time after such spontaneous reactivation, their first response was a note OFF message on release, and after that they could be pressed and send midi ON and released and send midi OFF.
3) that sometimes when working properly it can spontaneously generate a mess and then stop working: I was playing in Analog Lab 2, but suddenly it started changing patch spontaneously, as if someone were turning knob #1 in random directions. Then after quite a bit of this behaviour, the well known keys stopped working. Fortunately MIDI Monitor was active so I have a log of the whole process (incoming and outcoming MIDI messages).