For people encountering the same issue
The above code works as expected. The solution was getting the stuff behind the scenes right. Make sure, that the instance of the ViewModel which is the value of the property you want to bind to is definitely contained in the CollectionViewSource.
In my case the issue was deserializing an object tree incorrectly, so objects were instantiated twice. Then for each object a distinct ViewModel was initialized and then obviously the value of the property was not contained in the list.
Remark
To check whether this is an issue in your case, you can try the following:
Override the ToString() methods of the ViewModels displayed in the ComboBox like this:
public override string ToString()
{
return "VM"+ Model.GetHashCode().ToString();
}
Then you can easily compare the items in the source collection with the value on your property. Not the most professional way, but it did the job for me.