InvalidOperationException Ellipse not Found after Binding "IsChecked" - wpf

Question

I am using a Togglebutton Style from here after Binding the IsChecked Property to my ViewModel the Program crashes with a System.InvalidOperationException Ellipse not found in "System.Windows.Controls.ControlTemplate".
The Style looks like this:
<Style TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Viewbox>
<Border x:Name="Border"
CornerRadius="10"
Background="#FFE2E2E2"
Width="40"
Height="20">
<Border.Effect>
<DropShadowEffect ShadowDepth="0.5"
Direction="0"
Opacity="0.3" />
</Border.Effect>
<Ellipse x:Name="Ellipse"
Fill="#FF909090"
Stretch="Uniform"
Margin="-8 -4"
Stroke="Gray"
StrokeThickness="0.2"
HorizontalAlignment="Stretch">
<Ellipse.Effect>
<DropShadowEffect BlurRadius="10"
ShadowDepth="1"
Opacity="0.3"
Direction="260" />
</Ellipse.Effect>
</Ellipse>
</Border>
</Viewbox>
<ControlTemplate.Triggers>
<EventTrigger RoutedEvent="Checked">
<BeginStoryboard>
<Storyboard>
<ColorAnimation Storyboard.TargetName="Ellipse"
Storyboard.TargetProperty="(Ellipse.Fill).(SolidColorBrush.Color)"
To="#00facc"
Duration="0:0:0.05"
AccelerationRatio="0.7"
DecelerationRatio="0.3" />
<ThicknessAnimation Storyboard.TargetName="Ellipse"
Storyboard.TargetProperty="Margin"
To="20 -4 -8 -4"
Duration="0:0:0.15"
AccelerationRatio="0.7"
DecelerationRatio="0.3" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
<EventTrigger RoutedEvent="Unchecked">
<BeginStoryboard>
<Storyboard>
<ColorAnimation Storyboard.TargetName="Ellipse"
Storyboard.TargetProperty="(Ellipse.Fill).(SolidColorBrush.Color)"
To="#00facc"
Duration="0:0:0.05"
AccelerationRatio="0.7"
DecelerationRatio="0.3" />
<ThicknessAnimation
Storyboard.TargetName="Ellipse"
Storyboard.TargetProperty="Margin"
To="-8 -4"
Duration="0:0:0.15"
AccelerationRatio="0.7"
DecelerationRatio="0.3" />
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
And my usage like this:
<ToggleButton Grid.Column="0"
Grid.Row="2"
IsChecked="{Binding IsAutostop}"/>
I'd like to use the style AND be able to use the "IsChecked" Property.
EDIT
Thanks for the comments.
I open the window with a Service bound to a command in a taskbaricon.
The Service:
class WindowService
{
public void OpenOptionsView(MainViewModel mvm)
{
OptionsView optionsView = new OptionsView { DataContext = mvm };
optionsView.Show();
}
}
The important parts of the MainviewModel:
public ICommand OpenOptionsViewCommand { get; private set; }
public bool IsAutostop { get { return Settings.Default.Autostop; } set { Settings.Default.Autostop = value; Settings.Default.Save(); OnPropertyChanged(); } }
public MainViewModel()
{
OpenOptionsViewCommand = new RelayCommand(OpenOptionsView, param => true);
}
private void OpenOptionsView(object o)
{
WindowService ws = new WindowService();
ws.OpenOptionsView(this);
}
I stored my TaskbarIcon in a seperated View:
<tb:TaskbarIcon>
<tb:TaskbarIcon.ContextMenu>
<ContextMenu Background="#1e1e1e">
<MenuItem Header="Programm öffnen"
Foreground="White"
Command="{Binding OpenOptionsViewCommand}"/>
</ContextMenu>
</tb:TaskbarIcon.ContextMenu>
</tb:TaskbarIcon>
EDIT II:
I just found out, that the problem occurs when the bound value is True and the window should be opened with a checked Togglebutton.