To reproduce:

radRichTextEditor.Document = new RadDocument();
radRichTextEditor.Document.MeasureAndArrangeInDefaultSize();
radRichTextEditor.UpdateEditorLayout();
radRichTextEditor.Document.CaretPosition.MoveToLastPositionInDocument();

var p = new Paragraph();
p.Inlines.Add(new Span("Cell"));
var cell = new TableCell();
cell.Blocks.Add(p);
var row = new TableRow();
row.Cells.Add(cell);
var table = new Table();
table.Rows.Add(row);
var section = new Section();
section.Blocks.Add(table);
radRichTextEditor.Document.Sections.Add(section);
radRichTextEditor.Document.CaretPosition.MoveToLastPositionInDocument();

var provider = new RtfFormatProvider();
var txt = provider.Export(radRichTextEditor.Document);
Clipboard.SetText(txt, TextDataFormat.Rtf);

Workaround: measure the document after the section is added:
radRichTextEditor.Document.Sections.Add(section);
radRichTextEditor.Document.MeasureAndArrangeInDefaultSize();
radRichTextEditor.UpdateEditorLayout();