I'm using an Xbox One controller for Windows. Whenever I try to rebind a button in the Witcher 2 Configuration tool (for example, change D-Pad Up from Adrenaline to Igni), it (obviously) prompts that the key is already assigned, but when I click to continue anyway, the OK button just greys out, leaving me unable to save any changes I wanted to be made. Even clicking "Restore Defaults" after making some changes doesn't make the OK button clickable. Would the only solution to this be modifying the User.ini file, or am I missing something?

Have you already tried running the config tool explicitly as an Administrator? (right-click it and use the "Run as" option)

I did, but that doesn't seem to have any effect.

Hm, at least you can still edit the game's ini file, but you should back the altered version up as the game might overwrite it with its defaults.

Here's an extensive thread about gamepad re-binding:
http://forums.steampowered.com/forums/showthread.php?t=2658771

Thanks, I suppose it's the only way out of this situation then. The thing is, I tried rebinding keyboard controls which didn't disable the OK button, so that's even more strange. Maybe the Xbox One controller is the problem?

It's not xbox one pad issue. In my game client problem concerns all kind of joypads (ITOH, changing keyboard settings works).

I give up and follow the instruction for user.ini.