I downloaded the backup installer of the game (I don't use Galaxy), and installed it without issues (Windows 8.1), but if I run KnightShift from my regular Windows user account the game prompts me to enter the serial key. Apparently, since I had to run the installer with Admin privileges, it "registered" the game for the Admin account only, because I can access the game menu and play if I run it as an administrator.

This of course is an undesired state of affairs, no way I'm playing a game that has Internet access as an Admin.