Using Webservice does not fill ListView or Picker

Hi Guys,

Hope you could explain, what I'm doing wrong. Working on this since 2 days and it feels like I'm getting depressive .
My Webservice is doing fine. When I start the debugger I see, that my ObservableCollection Person get filled with data. But the ListView and Picker are still empty.

class HomeViewModel : INotifyPropertyChanged
    {

        public ObservableCollection<Person> Persons { get; set; }

        public HomeViewModel()
        {
            LoadPersons();
        }

        public  void SavePerson(string firstname, string lastname)
        {

        }
        private async void LoadPersons()
        {
            var personServices = new PersonServices();
            Persons = new ObservableCollection<Person>(await personServices.GetPersons());

        }

        public event PropertyChangedEventHandler PropertyChanged;

    }

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="debtMobileApp.view.HomePage"
             xmlns:ViewModels="clr-namespace:debtMobileApp.ViewModel">
    <ContentPage.BindingContext>
        <ViewModels:HomeViewModel/>
    </ContentPage.BindingContext>


    <ContentPage.Content>
        <StackLayout>
            <Label Text="Debt App Home" FontSize="Large" HorizontalOptions="Center"
                    FontAttributes="Bold" />
            <Entry Placeholder="Betrag" Keyboard="Numeric"></Entry>
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"></RowDefinition>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*"></ColumnDefinition>
                    <ColumnDefinition Width="*"></ColumnDefinition>
                </Grid.ColumnDefinitions>

                <Label Text="Ausgabe" x:Name="lblSwitch" Grid.Row="0" Grid.Column="0"></Label>
                <Switch Toggled="ActionToogleEA" Grid.Row="0" Grid.Column="1" ></Switch>
            </Grid>
            <Picker x:Name="personPicker" Title="bitte Person auswählen" ItemsSource="{Binding Persons}" ItemDisplayBinding="{Binding Firstname}">
            </Picker>
            <Button Text="speichern"></Button>
            <ListView x:Name="DebtListView" ItemsSource="{Binding Persons}">
                <ListView.ItemTemplate>
                    <DataTemplate>
                        <ViewCell>
                            <StackLayout>
                                <Label Text="{Binding Firstname}"></Label>
                                <Label Text="{Binding Lastname}"></Label>
                            </StackLayout>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

Binding seems to be ok, cause when I change my LoadPersons() method to

  private async void LoadPersons()
                {
                    Persons = new ObservableCollection<Person>();
            Persons.Add(new Person{ID=1, Firstname="first", Lastname="last"});

                }

it works.

How can this be, even the Person List gets filled by using the Webservice? Deserialization works as well.
This is where I call the Rest Service.

public async Task<List> GetPersons()
{
List persons = new List();
/*
persons.Add(new Person { ID = 1, Firstname = "Alf", Lastname = "Luri" });
persons.Add(new Person { ID = 1, Firstname = "Mia", Lastname = "Meyr" });
persons.Add(new Person { ID = 1, Firstname = "Sepp", Lastname = "Karlson" });
persons.Add(new Person { ID = 1, Firstname = "Sepp", Lastname = "Karlson" });

        */
        string restUri = "http://debtapp.ideflix.at/api/Person";

         var uri = new Uri(restUri);

         var response = await client.GetAsync(uri);
         if (response.IsSuccessStatusCode)
         {
             var content = await response.Content.ReadAsStringAsync();
             persons = JsonConvert.DeserializeObject<List<Person>>(content);
         }
        return persons;
    }