C# WPF ComboBox Displayed Value = "" / Which Events?

I'm struggling with a small Problem:

I've built a ComboBox which is a littlebit Special:

                    <ComboBox x:Name="selBenutzer"  SelectionChanged="selBenutzer_SelectionChanged"  Grid.Row="1" Height="30" Width="150" IsEditable="True" IsReadOnly="True" Text="Mitarbeiter"  >
                        <ComboBox.ItemTemplate>
                            <DataTemplate>
                                <StackPanel Name="StackPanelSelBenutzer" Orientation="Horizontal" MouseLeftButtonDown="SelBenutzer_MouseLeftButtonDown">
                                    <CheckBox Name="CheckBoxSelBenutzer" IsChecked="{Binding IsSelected}" Width="20" Checked="CheckBoxSelBenutzer_Checked" Unchecked="CheckBoxSelBenutzer_Unchecked"/>                                        
                                    <TextBlock Name="TextBlockSelBenutzer" Text="{Binding Value}" Width="100" />
                                </StackPanel>
                            </DataTemplate>
                        </ComboBox.ItemTemplate>                            
                    </ComboBox>

I needed a ComboBox for some Employees. But i needed to have CheckBoxes inside my Combobox (next to the TextBlock-UI for displaying the Item Name), so i could make a Multiselect. The Checked-/Unchecked-Events get the Text Property of the Checked-/Unchecked-Item and add it to a List. Then the List will be shown Comma-Separated in The .Text Propertie of the ComboBox..

  private void CheckBoxSelBenutzer_Checked(object sender, RoutedEventArgs e)
  {
      CheckBox cb = (CheckBox)sender;
      StackPanel sp = (StackPanel)cb.Parent;
      TextBlock tb = (TextBlock)sp.Children[1];
      checkedUsersList.Add(tb.Text);             // Unckecked: .Remove(tb.Text)
      var result = String.Join(", ", checkedUsersList.ToArray());
      selBenutzer.Text = result.ToString();
  }

So this works very well.. But at then i got a Problem: What if you not hit the CheckBox? If i hit the TextBlock-UI instead of the Checkbox it not shows the Values I wanted to show and also not hit the CheckBox..

I got the Key and Value Pair from the Collection which was used as Source. So I've built a LeftMouseClickEvent on my StackPanel:

    private void SelBenutzer_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {

        StackPanel sp = (StackPanel)sender;
        CheckBox cb = (CheckBox)sp.Children[0];
        if (cb.IsChecked == true)
        {
            cb.IsChecked = false;
        }
        else
        {
            cb.IsChecked = true;
        }            
    }

So now I was able to Klick on the TextBlock and the Checked-Property was updated automatically because of my MouseLeftButtonClick-Event.

But now when i just click on the TextBlock it first shows me the Value + Key -Pair and if i open the ComboBox again and Select another TextBlock the ComboBox.Text-Propertie is like an empty String?

I tried many Events but i think one last Event is always fired by the ComboBox automatically. But i don't know why and how it has changed?

I would be glad about every help