Ah, found it 
Read Holding Registers (3) was the reason why it wasn’t working before.
Now I’m able to transmit the status of the digital input to our MQTT Broker 
Remarks
I have configured the Data to Server to have 3 sections in the MQTT topic (see screenshot above). I noticed that the MQTT message with the 3 sections is immediatly followed by another message with only 2 sections. The Modbus section is gone. Do you have an idea why?