Elusive WPF Exception related to ToolTip - wpf

Question

I have deployed a version of my (rather complex and sizeable) WPF Application and a few users (not all of them) encounter the following Exception.
FEHLER: Bei dem angegebenen Element handelt es sich bereits um das logische untergeordnete Element eines anderen Elements. Führen Sie zuerst eine Trennung durch.
bei System.Windows.FrameworkElement.ChangeLogicalParent(DependencyObject newParent)
bei MS.Internal.FrameworkObject.ChangeLogicalParent(DependencyObject newParent)
bei System.Windows.FrameworkElement.AddLogicalChild(Object child)
bei System.Windows.Controls.ContentControl.OnContentChanged(Object oldContent, Object newContent)
bei System.Windows.Controls.ContentControl.OnContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
bei System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
bei System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
bei System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
bei System.Windows.Data.BindingOperations.SetBinding(DependencyObject target, DependencyProperty dp, BindingBase binding)
bei System.Windows.Controls.PopupControlService.RaiseToolTipOpeningEvent()
bei System.Windows.Controls.PopupControlService.OnRaiseToolTipOpeningEvent(Object sender, EventArgs e)
bei System.Windows.Threading.DispatcherTimer.FireTick(Object unused)
bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
The english version of the exception message is:
Specified element is already the logical child of another element. Disconnect it first.
My main Problem is,
a) that I am not able to reproduce this on my development machine.
Which does not mean it does not occur under the same circumstances, rather the users all say independently that the Exception occurs at no specific point in time, action or state of the software.
b) none of the above calls are actually referring to my own code.
So I guess it must be somewhere in the XAML
I am completely lost and in hope of some pointers in a rough direction where to look.
EDIT: I have now been able to reproduce and isolate the problem
Demo Application
The Demo Application reproduces an Exception that will occur when a complex ToolTip (anything but a String Object) is placed on an Element (Button) which is in turn placed on a Popup.
<Grid HorizontalAlignment="Center" VerticalAlignment="Center">
<ToggleButton x:Name="_PopUpToggle" Content="Open Popup"/>
<Popup
IsOpen="{Binding ElementName=_PopUpToggle,Path=IsChecked}"
Placement="Bottom"
>
<Grid Background="Wheat">
<StackPanel
HorizontalAlignment="Center"
VerticalAlignment="Center">
<Button
Content="StackPanelToolTip"
Click="Button_Click">
<Button.ToolTip>
<StackPanel Orientation="Horizontal">
<TextBlock Text="Not "/>
<TextBlock Text="Plain"/>
</StackPanel>
</Button.ToolTip>
</Button>
</StackPanel>
</Grid>
</Popup>
</Grid>
To reproduce start the application, open the Popup, mouseover the first button, wait until the ToolTip appears and click. Repeat. This produces the following Exception with the corresponding TraceStack:
Bei dem angegebenen Element handelt es sich bereits um das logische untergeordnete Element eines anderen Elements. Führen Sie zuerst eine Trennung durch.
bei System.Windows.FrameworkElement.ChangeLogicalParent(DependencyObject newParent)
bei System.Windows.FrameworkElement.AddLogicalChild(Object child)
bei System.Windows.Controls.ContentControl.OnContentChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
bei System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
bei System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, OperationType operationType)
bei System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
bei System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
bei System.Windows.Data.BindingOperations.SetBinding(DependencyObject target, DependencyProperty dp, BindingBase binding)
bei System.Windows.Controls.PopupControlService.RaiseToolTipOpeningEvent()
bei System.Windows.Threading.DispatcherTimer.FireTick(Object unused)
bei System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
bei System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
The english version of the exception message is:
Specified element is already the logical child of another element. Disconnect it first.
Binding the ToolTip to a plain String Object (try the second Button) does work without any issue. This is my current workaround.

Answer 1

As pointed out by Mathew, enclosing the Complex Content of the ToolTip-Property in <ToolTip></ToolTip> resolves the issue.