Do a fresh install of GOG NWN, hit the self-extracting archive and pointed it to the clean NWN install folder(C:\GOG Games\NWN Diamond\ is default) and let it work. Copy your old nwnplayer.ini and nwncdkey.ini(or get your GOG keys and put them in here) into the folder and run the modded .exe(nwmain-169-hg.exe)
The loader itself isn't the problem. The problem I seem to recall arises from nwmain.exe having code that fails when nwmain.exe is itself named something else. This may, however, be the exe looking for itself to load compiled-in resources at some point, in which case it may not be an issue as long as nwmain.exe exists in the same folder.
I did the re-naming thing too, while trying to get HGX to run with ARF2. I'm guessing the changes to the exe in the modified one moved the hooks HGX needs because I never did get them to play together.
Now I just use ARF2 to see how long servers have been up, then log in with HGX.