Thank you @scook for your help !
i tried a fresh install with the web installer. And i still have the same issue.
The "C:\Program Files\Cakewalk\Studio Instruments" and subdirectories are not compressed.
The "Boost11" VST wich is in the same folder as the Studio instruments is working fine.
I've compared access rights between "Boost11" directory/files and "SI-Drum Kit" and they are the same.
When i add a Studio instrument track, the instrument window is working, i can interact with it. But no sound is playing. Maybe we have a problem with the access to the wave files of the instruments.... (just a guess)
FYI, i also have a KORG synth wich is working fine.