Sensibility problem in just one key

Hello Beto,

If you have Pianoteq PRO, set the volume of the offending note ... rather high (or at least a value that will match other notes you intend on playing), and set the velocity sensitivity of that note ... rather low.  In this way, almost regardless of what your keyboard sends, you should obtain a rather even sounding note.

Cheers,

Joe

Most probably it's not the cable because a bad connection would affect other notes or MIDI data, too.
The sensors of common keyboards are not optical but just simple electrical contacts. Each key has two of them with a certain mechanical offset to each other. When a key is pressed, it first makes one contact and then the second one. The time between the contacts being made is inversely proportional to the speed of the key motion. If there is something wrong with one contact, the time cannot be gauged correctly, resulting in wrong velocity values. Maybe the first contact is dirty, so it becomes conductive too late, the time until the second contact is made is shorter, so the velocity is estimated too high.
Since this is a mechanical problem, you could try to resolve it yourself (if you think you're not all thumbs) by cleaning the corresponding contacts. Sometimes they are metal reeds, sometimes they are rubber contacts. It is also possible there is one make and one break contact. It is always a good idea to be careful and gather information first.