Some documents loaded through WordsProcessing and then exported to a RadFixedDocument by utilizing the ExportToFixedDocument() method, fails on export through ImageFormatProvider with System.AggregateException.

As a workaround, you can export the RadFixedDocument to a PDF and then import it again before exporting it to an image.

var pdfFixedProvider = new Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.PdfFormatProvider();
bytes = pdfFixedProvider.Export(fixedDocument);
fixedDocument = pdfFixedProvider.Import(bytes);