I'm sorry, but it's very difficult for me to help you with only these details. Can you even tell, what kind of files you were replacing? Only image files?
I'm speculating that FeralHeart tries to create a GUI object named CharCreateScreen/SX11, that uses 1 or more image files to work. If you deleted or modified any of the images, the creation of the GUI object may fail and that could explain the error message.
Although I believe that at this point it may be easier for you to just reinstall whole game.