.Net Core 3.1 using GraphQLHttpClient from C#/WPF gets stuck - but works with ConsoleApp

Question

I have created a small GraphQl server - that is working very nicely from Banana Cake Pop.

Then I have created a small C#/.Net Core 3.1 WPF App - that has a Class Library that uses GraphQL.Client 3.2.2 and GraphQL.Client.Serializer.Newtonsoft 3.2.2

It also has a class like this to call the GraphQL Endpoint:

    public class RepositoryBase
{
    protected GraphQLHttpClient GraphQLHttpClient { get; private set; }

    public RepositoryBase()
    {
        GraphQLHttpClient = new GraphQLHttpClient("http://xxx/graphql/",
                                                    new NewtonsoftJsonSerializer());
    }
}

public class ArtRepository : RepositoryBase
{
    public List<Art> GetArterByGruppe(short GruppeId)
    {
        var queryString = @"query { arter(where: { and:[{ gruppeId: { eq: 1200} } { su: { eq: false} } { setIDk: { eq: true} }]}
                      order: {artNavn:ASC}) {
                      artNavn 
                      artId 
                      gruppe {
                          gruppeNavn 
                          gruppeId 
                      } 
                    }
                }";

        var request = new GraphQLRequest
        {
            Query = queryString
        };

        var result = GraphQLHttpClient.SendQueryAsync<ArtResponse>(request).ConfigureAwait(false);
        return result.GetAwaiter().GetResult().Data.Arter;
    }
}

If I call this method from the WPF App - like in the Windows Loaded event - the call to the GrapQl Server gets stuck - with no errros.

            var repository = new ArtRepository();
        var x = repository.GetArterByGruppe(1700);

So - in dispair - I created a ConsoleApp project in the WPF solution - that makes the same call to the ArtRepository Class - and it just runs very fine and returns data.

I don't understand how the WPF App can influence the GraphQL client in a way that it get's stuck?