I found the error myself.
I have to specify the assembly also for some reason.
It wasn't required in app.xaml.
<resourcedictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:src="clr-namespace:Preparation.DataModel;assembly=Preparation"
xmlns:conv="clr-namespace:Preparation.Converters;assembly=Preparation"
xmlns:viewmodel="clr-namespace:Preparation.ViewModel;assembly=Preparation"
></resourcedictionary>
Maybe there is a setting somewhere else that I don't know about.