Hi,
My Keylab 88 MKII regularly has this problem, different keys, and it takes 2 sec. to fix it.
I got a 10-second video from Arturia tech support, that shows how you just have to pull the key out from the keyboard (toward your body) about 1/8 to 1/4 inch, and then push it back in.
It works great for me, hopefully will work for you.
Here is a link to the video:
https://www.dropbox.com/s/sc8f58pfiin1ihg/KL88_-_How_to_unlock___lock_a_key.mp4?dl=0In the video he pulls it out with pressure from his thumb. I find it easier to depress the keys on either side (with the other hand) and pull out gripping both sides of the key.
Very simple once you've done it. I don't know why the keys occasionally require this, but now that I know it doesn't bother me at all.
Note to anyone else reading--
this is for Keylab 88 MKII only. It does not work with 61 or 49-key models. I've seen someone report that it didn't work on the original Keylab 88 either.
Roger