The exception message is the following: System.ArgumentException: 'The value '102' is not a valid value for the enum 'FilterOperator'.'

To work this around, avoid using mixed typed values in the ItemsSource.

static MainWindow()
{
	EventManager.RegisterClassHandler(typeof(RadComboBox), RadComboBox.LoadedEvent, new RoutedEventHandler(OnComboBoxLoaded));
}

private static void OnComboBoxLoaded(object sender, RoutedEventArgs e)
{
	var comboBox = (RadComboBox)sender;
	if (!comboBox.IsEditable)
	{
		if (comboBox.IsEnabled)
		{
			var buttonChrome = comboBox.FindChildByType<ButtonChrome>();
			buttonChrome.Background = Brushes.Purple;
		}
		else
		{
			comboBox.Background = Brushes.Purple;
		}                
	}	
}

public class CustomRadComboBox : RadComboBox
    {
        protected override bool HandleKeyDown(Key systemKey, int platformKeyCode)
        {
            if(systemKey == Key.Oem5)
            {
                return false;
            }
            return base.HandleKeyDown(systemKey, platformKeyCode);
        }
    }

var template = this.comboBox.Template;
this.comboBox.Template = null;
this.comboBox.Template = template;

            <telerik:RadComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

            <ComboBox Margin="5"
                                 ItemsSource="{Binding Path=AvailableServices}"
                                 IsEditable="True" 
                                 Text="{Binding Path=Service, Mode=TwoWay}"/>

public class ViewModel
    {
        public ObservableCollection<string> AvailableServices { get; set; } = new ObservableCollection<string> { "UPS Standard", "UPS Express Saver", "UPS Express 12:00", "UPS Express" };

        public string Service { get; set; } = "UPS Express";
    }

<telerik:RadComboBox x:Name="comboBox"
                     AllowMultipleSelection="True">
    <telerik:RadComboBox.MultipleSelectionBoxTemplate>
        <DataTemplate>
            <ItemsControl ItemsSource="{Binding ElementName=comboBox, Path=SelectedItems}"
                          AlternationCount="{Binding RelativeSource={RelativeSource Self}, Path=Items.Count}">
                <ItemsControl.ItemsPanel>
                    <ItemsPanelTemplate>
                        <StackPanel Orientation="Horizontal"/>
                    </ItemsPanelTemplate>
                </ItemsControl.ItemsPanel>
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <StackPanel Orientation="Horizontal">
                            <TextBlock x:Name="commaTextBlock" Text=", "/>
                            <TextBlock Text="{Binding}" HorizontalAlignment="Left"/>
                        </StackPanel>
                        <DataTemplate.Triggers>
                            <Trigger Property="ItemsControl.AlternationIndex" Value="0">
                                <Setter Property="Visibility" TargetName="commaTextBlock" Value="Collapsed" />
                            </Trigger>
                        </DataTemplate.Triggers>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </DataTemplate>
    </telerik:RadComboBox.MultipleSelectionBoxTemplate>
</telerik:RadComboBox>