You don't say what version of Ghostscript you are using, and because you are using something which executes Ghostscript you haven't given me a command line I can actually try.
However, on the assumption that the (ImageMagick?) command line in the error is the one being used, I rendered your PDF file to PNM without problems using a copy of the current code, so I suspect that this is resolved, I would recommend you upgrade to the current version.
If this doesn't solve the problem, then you should open a bug report at http://bugs.ghostscript.com please attach the PDF file there, and supply a Ghostscript command line which demonstrates the problem.