It seems ImageMagick is raising an exception. I will make sure this exception is caught an raised properly before the next release. You need to read the image before you add the profile and change the colorspace.
using (MagickImage image = new MagickImage(oldFile, mySettings)) { image.AddProfile(new ColorProfile(Server.MapPath("~/ColorProfile/CMYK/USWebCoatedSWOP.icc"))); image.ColorSpace = ColorSpace.CMYK; image.Write(newFile); }