I am also using 9.15 and I can process the image without any problems. Is it possible that you have two versions of Ghostscript installed (x86 and x64)? Maybe Magick.Net/ImageMagick is using another version?
p.s. I will improve the error reporting to also report the version of Ghostscript in the next version of Magick.NET
p.s. I will improve the error reporting to also report the version of Ghostscript in the next version of Magick.NET