Hi Steve,
I have worked with the scenario shared by you and have worked in Windows Server 2008 R2 environment and have successfully generated the PDF using Aspose.Slides for .NET4.0 5.7.0. Our development team also have not been able to reproduce the issue. For your kind reference, I have shared the complete project code along with generated out put and system information with you. Please share, if I may help you further in this regard.
Many Thanks,