Validation errors disappear when tabs are changed in AvalonDock. Therefore, I need to change tab content template in AvalonDock to wrap the content with AdornerDecorator. But I don't know how to do this.
I solved this problem, but do not delete the question since it can be useful for others. This is the solution:
<ad:DockingManager>
<ad:DockingManager.LayoutItemTemplate>
<DataTemplate>
<AdornerDecorator>
<ContentControl Content="{Binding}"/>
</AdornerDecorator>
</DataTemplate>
</ad:DockingManager.LayoutItemTemplate>
</ad:DockingManager>