It isn't installed in the programm files so I'll give ist a try with only turning off the UAC. Hopefully it works then
I don't think that will help you. I can get through the OC with UAC on default setting.
If UAC has any effect at all, it will be during the installation program, so you would need to uninstall the game, disable UAC & your antivirus software while installing the game, & reinstall the game not in program files & running the setup program as Administrator.
If you use AVG antivirus, you need to disable "AVG self protection" (permanently)
Then check your C:\Users\username\Documents\Neverwinter Nights 2 directory & make sure your username has full control (& not just system).
You will still need to save often, the OC is still buggy even if installed properly.
Check your event viewer for any errors that occurred while you were playing the game.