I solved it now using Converters for "Text" and "FontWeight" of the TextBoxes in the Expander.Header binding to the Name of the Group like this:
<TextBlock x:Name="ExpanderHeader3" Width="100" FontWeight="{Binding Path=Name,Converter={StaticResource fontWeightConverter}}" Text="{Binding Path=Name,Converter={StaticResource nameToActiveConverter}}"/>
The Converters go just through my database search for the "Name" and count the rows where "state"==2. Then they return the respective
count or
FontWeight.
Not quite elegant nor efficient, because I loop over every row of the underlying database, but it does what it is supposed to.