Hi mate, thank you for your reply. I know it may take time but I never saw a firmware upgrade taking more than 2 hours.... My fault.
Unfortunately right now the stick is not recognized anymore neither from Windows nor from Linux. It looks like I lost the US2Serial interface....
I have I ticket opened with the huys at z-wave.me, but meanwhile I bought a SiliconLab Z-Wave 700 stick, so I can try to brick also this one 😄