arthur.clg17: When the game is properly installed, a registry key is created and this error never happens.
For some reason your registry key is missing, my guess would be that you moved the game files manually from one machine to another.
At any rate, it's easy to fix, simply create the registry key yourself.
For a 32bit OS, create this registry branch: HKEY_LOCAL_MACHINE\SOFTWARE\City Interactive\Sniper2
If on a 64bit OS, then the location changes: HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\City Interactive\Sniper2
Inside the branch, create a string value named "serialnumber" and leave it blank.
That's it, problem should be solved now.
This works fine. I installed Ghost Warrior by the offline files, cause of problems with gog galaxy. When i create the registry key, the game no longer ask for the serial key.