When I export slides as images, when a slide contains smart art it causes and exception in the library.
I have observed the information shared by you and request you to please first try using Aspose.Slides for .NET 17.8 on your end. If there is still an issue then please share the working sample project along with source presentation reproducing the issue on your end.
The issue still exists and is not fixed in 17.9 either.
I have observed your comments. Can you please try to use Aspose.Slides latest version 18.10 on your end and share source files with sample code of there is still an issue.