public class Activity{
  public int Id { get; set; }
  public string Title { get; set; }
  public Task Task { get; set; }
}

public class Task{
  public int Id { get; set; }
  public string Title { get; set; }
}

public ApprovalViewModel{
  public ObservableCollection<Activity> Activities { /* ... property logic */ }
  public ObservableCollection<Task> Tasks { /* ... property logic */ }
}

<DataGrid ItemsSource="{Binding Activities}" AutoGenerateColumns="False">
    <DataGrid.Resources>
        <DataTemplate x:Key="displayTemplate">
            <TextBlock Text="{Binding Task.Title}"/>
        </DataTemplate>
        <DataTemplate x:Key="editTemplate">
            <ComboBox ItemsSource="{Binding Tasks}" <!--I think the problem is here-->
              SelectedValue="{Binding Task}"
              DisplayMemberPath="Title"/>
        </DataTemplate>
    </DataGrid.Resources>
    <DataGrid.Columns>
        <DataGridComboBoxColumn DisplayMemberPath="Title"/>
        <DataGridTextColumn Binding="{Binding User}" Header="User"/>
        <DataGridTextColumn Binding="{Binding Task.Project.Title}" Header="Project"/>
        <DataGridTemplateColumn
            Header="Task"
            CellTemplate="{StaticResource displayTemplate}"
            CellEditingTemplate="{StaticResource editTemplate}"/>
        <DataGridTextColumn Binding="{Binding Description}" Header="Description"/>
    </DataGrid.Columns>
</DataGrid>

<ComboBox
  ItemsSource="{Binding Path=DataContext.Tasks,
                        RelativeSource={RelativeSource FindAncestor,
                                        AncestorType={x:Type UserControl}}}"
  SelectedValue="{Binding Task.Title}"
  SelectedValuePath="Title"
  DisplayMemberPath="Title"/>

ItemsSource="{Binding Path=DataContext.Tasks, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"