Umberto,
Please see below in red...
The problem simply cannot happen, under normal circumstances, and it's unlikely to be caused by another simulator addon There is no other scenery add-on installed. I intentionally chose to isolate GSX2. Instead, the only possible reason for this is some other software you installed when you reinstalled the OS, which is interfering with the registration. This is a Windows 10 reset, wouldn't the registry be clean? The only products I installed were Active Sky and Ultimate Traffic Live
The most likely cause is the antivirus, so the first thing to try would be acting on it, and disabling the whole Addon Manager folder from scanning. I haven't installed any yet. Windows Defender is the only product installed (its not, as you know, a user option to not install). Ideas on how to disable?
Another possible reason is that you created a limited-permission user account when installing the OS. It's best if you run the sim from an user account with full privileges. I believe I do have full privileges, how do I verify?
Try running the sim As Admin, at least to do the activation. If that doesn't work, try changing the properties for this file:
Program Files (x86)\Addon Manager\Couatl\Couatl.exe
to run as Admin. Right-click on it, select Properties->Compatibility and check the "Run as administrator" option, then start the sim again and see if the activation now sticks.