To reproduce use the following code:

TxtFormatProvider txtProv = new TxtFormatProvider();
radRichTextBox1.Document = txtProv.Import("Ein erster Text");

radRichTextBox1.ChangeListStyle(DefaultListStyles.Numbered);

ListLevelStyle style = new ListLevelStyle();
style.FontFamily = "Segoe UI";
style.NumberingFormat = ListNumberingFormat.Decimal;

RtfFormatProvider rtfProv = new RtfFormatProvider();
string txtRtf = rtfProv.Export(radRichTextBox1.Document);

Workaround:

radRichTextBox1.Document = txtProv.Import("Ein erster Text");
ListStyle style = new ListStyle();
style.Levels.Add(new ListLevelStyle() { FontFamily = "Segoe UI", NumberingFormat = ListNumberingFormat.Decimal, LevelText = "1"});

radRichTextBox1.ChangeListStyle(style);

Resolution: 

This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.