s88 owners If one of your keys are not responding correctly, this is how I fixed mine

Without opening the board. for clarification Komplete kontrol s88
My G#2 stopped responding on normal playing also the light guide for that key would not light up when pressed, then after much fiddling I got it to respond but it was only outputing maximum velocity even when played softly, the light also lit up when the note did sound.  I suspect I let the temp in my control room get too cold followed by a blast of heat creating condensation.  I found one other person reporting this behavior with a fatar keybed controller and he opened his board and fixed the problem by getting down to the little rubber/silicon pressure boots that's under the keys and he removed them and cleaned them with alcohol and q-tips.  I did the same thing but without opening the board.  trying to diagnose the problem I discovered that (turn power off and/or unplug board)
 
If it's a black key grab it by the sides firmly and pull it toward you until it unlocks
then start pulling up gently until it won't go any higher, the key sits about  1/2" higher at this point
with one hand holding the key ready to remove it, take the other hand and get your finger tips under the light guide and lift it
just enough for the key to come right out, and it comes right out easily at that point
 
If it's a white key,  push the key on the left and right of it down so you can grab it the same way as the black key
pull it towards you until it unlocks, this time you need to get your finger tips in that space on the front of the keybed that's 
closest to you, and pull open just enough to get the key to lift past the lip inside and now the key should be sitting up about
a 1/2" like the black key was, lastly while holding the key ready to remove lift the light guide just enough and it comes right out.
 
I know this is all scary and stuff but I did this process over and over as I'm typing this to confirm it works and doesn't break anything.  I am trying to be as detailed as possible so this writing may look long but I can remove a key in 3 seconds now that I'm used to doing it.
 
Which ever key is your problem key (hopefully nothing too close to the ends because I haven't tried those but they appear to require more force as the light guide is not so flexible at the ends) remove it and two keys to the left and two keys to the right, place them aside in a manner that you know exactly which order to put them back in. you will see now that there is a continuous rubber/silicon boot strip under the keys that you now have to oh so carefully lift up out of it's seat but not remove completely
this was the scariest part trying not to damage this strip, so be very careful and take your time on this part, I used a wood shim that I resized the width so it could fit between the little "anchors" that is holding this strip in place.  these anchors are just tiny peg like spikes that are pressed into the pcb, use patience and work them out little by little until it's out of the holes that holding them in focusing on the problem key area.  once you have enough space to get a q-tip in there that's enough, dip the q-tip in some rubbing alcohol and clean both the bottom of the rubber strip and the contacts on the pcb, do this twice, I expected to see some sort of residue come off on the q-tips but it was very clean, which is why I suspect moisture being the only problem.  let it dry out for a couple of minutes and push the rubber strip back into the holes until it is completely re-seated and tight to the board just like it was before you removed it. It won't just snap in, you have to work it in just like you worked it out.  put the keys back in by following the instructions in reverse for the respective keys.
 
Initially I opened a support ticket with NI but they were taking too long and I was sure they were going to tell me my warranty has expired anyway.  They just got back with me today telling me my warranty is 1 year from purchase and can I send them  short video showing the problem.  problem solved so no to that. and I'm sure  I had this longer than a year.  It works flawlessly now.  If you have one of these controllers, I suggest you save this post in case your sad day comes and you want to be happy again like me.   
 
 

You're welcome.