I am in the same boat here. 6 devices, geographically dispersed connecting to 5 different gateways. All went dark approx 15h ago. I can see encrypted traffic from the devices hit the gateway, but no application data for a system that has been running for ~2 years. One device+gateway is sitting here in front of me, if I power cycle it I can see the join messages in the application console, then it joins and starts transmitting data but no data is reaching the application @ TTN.