Dynamic binding of combobox in MVVM, WPF and EF

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C#

WPF

combobox

MVVM

Binding

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Hello Community,
In my Application, I have two comboboxes. The second one depends on the value of the previous Selected Value. But the cb2 is not updated when I select something in cb1.
When I start my Application the cb1 is populated properly with all Clients that exist in my Database (Binding with EnitiyFramework works) and when I select a value there, the property "ClientForwarders" is updated to all forwarders that belong to the client selected, BUT the combobox does not show them.

My Model:

public class Client
    {
        public int ClientId { get; set; }
        public string ClientName { get; set; }

        public virtual ICollection<CommandMapping> CommandMappings { get; set; }
    }

 public class Forwarder
    {
        public int ForwarderId { get; set; }
        public string ForwarderName { get; set; }

        public virtual ICollection<CommandMapping> CommandMappings { get; set; }
    }

 public class CommandMapping
    {
        public int CommandMappingId { get; set; }
        public string CommandMappingName { get; set; }
        
        public virtual Client Client { get; set; }
        public virtual Forwarder Forwarder { get; set; }
    }

View:

<Window [...]>
	<Window.DataContext>
		<vm:MainWindowViewModel/>
	</Window.DataContext>
	<ComboBox Name="cb1" ItemsSource="{Binding Clients, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="ClientName"  SelectedItem="{Binding SelectedClient}"/>
	<ComboBox Name="cb2" ItemsSource="{Binding ClientForwarders, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="ForwarderName" SelectedItem="{Binding SelectedForwarder}"/>
</Window>

And my ViewModel:

namespace ProjectXY.ViewModel
{
    public class MainWindowViewModel : INotifyPropertyChanged
    {
        #region ClientHandling
        public IEnumerable<Client> Clients { get; private set; }
        private Client _selectedClient;
        public Client SelectedClient
        {
            get => _selectedClient;
            set
            {
                _selectedClient = value;
                OnPropertyChanged(nameof(SelectedClient));
            }

        }
        #endregion

        #region ForwarderHandling
        public IEnumerable<Forwarder> ClientForwarders { get; private set; }
        private Forwarder _selectedForwarder;
        public Forwarder SelectedForwarder
        {
            get => _selectedForwarder;
            set
            {
                _selectedForwarder = value;
                OnPropertyChanged(nameof(SelectedForwarder));
            }
        }
        #endregion

        private EFDataModel _ctx;

        public MainWindowViewModel()
        {
            LoadData();
        }

        protected void LoadData()
        {
            Refresh();
        }
        public void Refresh()
        {
            _ctx = new DataModel();
            _ctx.Clients.Load();
            _ctx.Forwarders.Load();
            _ctx.CommandMappings.Load();
            Clients = _ctx.Clients.Local.ToList();
        }

        public event PropertyChangedEventHandler PropertyChanged;

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            switch (propertyName)
            {
                case null:
                    return;
                case nameof(SelectedClient):
                    var forwarderIdList = new List<int>();
                    Task.WaitAll(_ctx.CommandMappings
                        .Where(cm => cm.Client.ClientId == SelectedClient.ClientId)
                        .ForEachAsync(x => forwarderIdList.Add(x.Forwarder.ForwarderId)));
                    ClientForwarders = _ctx.Forwarders.Local.Where(x => forwarderIdList.Contains(x.ForwarderId)).ToList();
                    
                    break;
            }
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

    }
}

What I have tried:

For Debugging I filled the "ClientForwarders" property directly in the "Refresh()" method. That leads to (as expected) all forwarders are being shown in cb2.