Steps to reproduce the issue:

1) Run the code below in IE11

2) Set background color to the "some text" cell by right clicking and then selecting cell property through the context menu.

3) Switch to design - there are many nbps's added.

ASPX:

<telerik:RadEditor ID="RadEditor1" runat="server">
            <Content>
                <table>
                    <tbody>
                        <tr>
                            <td>&nbsp;</td>
                            <td>&nbsp;</td>
                            <td>&nbsp;</td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>
                            <table>
                                <tbody>
                                    <tr>
                                        <td>&nbsp;</td>
                                        <td>&nbsp;</td>
                                        <td>&nbsp;</td>
                                    </tr>
                                    <tr>
                                        <td>&nbsp;</td>
                                        <td>some text&nbsp;</td>
                                        <td>&nbsp;</td>
                                    </tr>
                                    <tr>
                                        <td>&nbsp;</td>
                                        <td>&nbsp;</td>
                                        <td>&nbsp;</td>
                                    </tr>
                                </tbody>
                            </table>
                            &nbsp;</td>
                            <td>&nbsp;</td>
                        </tr>
                        <tr>
                            <td>&nbsp;</td>
                            <td>&nbsp;</td>
                            <td>&nbsp;</td>
                        </tr>
                    </tbody>
                </table>
            </Content>
        </telerik:RadEditor>
For the time being you can place the following JavaScript workaround below the editor's declaration:

        <script type="text/javascript">
            if ($telerik.isIE) {
                Telerik.Web.UI.Editor.CommandList.SetCellProperties = function (commandName, editor, commandArgs) {
                    var $ECL = Telerik.Web.UI.Editor.CommandList;
                    var argument = $ECL._getTableArgument(editor, 2, false, true);

                    if (!argument) {
                        alert(editor.getLocalizedString("cellwarning"));
                        return false;
                    }

                    $ECL._getDialogArguments(argument, "TABLE", editor, commandName);

                    var previousTable = $ECL._getParentTable(editor);

                    var callbackFunction = $ECL.getCallbackFunction(commandArgs, function (sender, args) {
                        $telerik.$(args.tableToModify).insertAfter(previousTable);
                        $telerik.$(previousTable).remove();

                        if (args.styleSheetToImport)
                            $telerik.$(args.styleSheetToImport).insertAfter(args.tableToModify);
                        if (args.styleSheetToRemove)
                            $telerik.$(args.styleSheetToRemove).remove();
                    });
                    editor.showDialog("TableWizard", argument, callbackFunction);
                    return false;
                }
            }
        </script>