I am unable to reproduce your reported issue with the latest version of Aspose.Slides for .NET v8.0.0. There is no issue in opening your attached PPTX file. I would suggest you to download and try the latest version and check if it works fine for you too. Hopefully, new version will resolve the issue at your end too.
I would not be able to use the latest version as my license subscription expired on Aug 1st 2013.
Aspose.Slides.Presentation pres = null;
//Create a new presentation based on a template.
pres = new Aspose.Slides.Presentation("asposefile.pptx");
Aspose.Slides.ISlide slides = pres.Slides.ToArray();
Aspose.Slides.ISlide sld = slides;
System.Console.WriteLine("About to Image...");
System.Drawing.Image slide = sld.GetThumbnail(1, 1);
pres = null;