I’m running the chirpstack-packet-multiplexer behind the Semtech UDP forwarder on a IMST IC880A-Gateway.
Everything works fine on the chirpstack side. I can see the join and uplink packets and also see the data (payload) on the Chirpstack-Application-Server. But on the TTN-side I only see the activation. I cant’t see any data (payload) on TTN. Without the multiplexer the packets are transmitted correctly to TTN an anything works fine on TTN-Side.
Something has to be set somewhere, for example on the TTN side, if the multiplexer is used?

Information about the problem described when using the multiplexer.
When I look at the gateway traffic in TTN, I see the information “router ttn-router-eu drop / reason: no brokers” in the uplink message. This means that no message is passed on to the TTN application.
Why is the message ‘dropped’? How can I change this behavior?

I don’t know, I guess you have to find out what router ttn-router-eu drop / reason: no brokers means.

Can I send a signal to the service to reload its configuration file? Can I have a default backend without specifying the (list of) gateway(s)?

You need to restart the Multiplexer, it does not auto-reload the config file on change, nor is there any signal to send to trigger a reload.