In most cases this error has to do with the file path either changing after it was embedded with the .acv curve or something bad in the file name of the .acv curve. With the file name/folders for the .acv files "spaces" are ok but "/" or "\" are not. The .acv files don't have the same kinds of problems with illegal file names in the qdif/.txt files. I am not sure if there is a character limit for the acv file name, but I might make sure that its name isn't too long.
Are you dragging the .acv file from a file system browser to the "GRAY_CURVE= " line or typing the file path manually?
Richard Boutwell