I did it another way, and finally got it working!!! (Ironically right after I posted this question... :P)
I changed my write app to:
DataWriter writer = new DataWriter();
writer.WriteString("Test");
proximityDevice.PublishBinaryMessage("Windows:WriteTag.Tietgen", writer.DetachBuffer(), new MessageTransmittedHandler((pDevice, id) =>
{
Debug.WriteLine("Message sent.");
}));
Then I just had to read it like this:
proximityDevice = Windows.Networking.Proximity.ProximityDevice.GetDefault();
proximityDevice.SubscribeForMessage("Windows.Tietgen", new MessageReceivedHandler((proxDevice, message) =>
{
Debug.WriteLine(message.DataAsString);
}));
And it worked :D