OK I finally arrived to the same error as you mentioned.
This is what I get from Datacake once it sends downlink
However if I schedule the same payload from Chirpstack GUI, it works fine. I can even see the data received on the node itself (RAK1070)
Also, in chirpstack logs I see status 200, unlike DataCake which tells that Chirpstack responded with 400
May 14 17:55:00 chirpstack[38585]: 2023-05-14T17:55:00.970192Z INFO chirpstack::integration::http: Posting event event=up url=https://api.datacake.co/integrations/lorawan/chirpstack/
May 14 17:55:00 chirpstack[38585]: 2023-05-14T17:55:00.971763Z INFO chirpstack::integration::mqtt: Publishing event topic=application/bd6d3d7f-c699-4ef9-8603-481aa9412675/device/ac1f09fffe08e82c/event/up
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.072377Z INFO up{deduplication_id=2e59b3ae-83d7-4d2f-b591-d960418472f2}:data_up: chirpstack::storage::downlink_frame: Downlink-frame saved downlink_id=1402343297
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.073174Z INFO up{deduplication_id=2e59b3ae-83d7-4d2f-b591-d960418472f2}:data_up: chirpstack::storage::device_session: Device-session saved dev_eui=ac1f09fffe08e82c dev_addr=01aa24aa
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.073266Z INFO up{deduplication_id=2e59b3ae-83d7-4d2f-b591-d960418472f2}:data_up: chirpstack::gateway::backend::mqtt: Sending downlink frame gateway_id=169bf3fffea9e42d topic=eu868/gateway/169bf3fffea9e42d/command/down json=true
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.148845Z INFO chirpstack::gateway::backend::mqtt: Message received from gateway region_config_id="eu868" topic="eu868/gateway/169bf3fffea9e42d/event/ack" qos=0 json=true
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.150006Z INFO chirpstack::storage::device_session: Device-session saved dev_eui=ac1f09fffe08e82c dev_addr=01aa24aa
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.150081Z INFO chirpstack::downlink::tx_ack: Log downlink-frame for gateway gateway_id=169bf3fffea9e42d
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.150821Z INFO chirpstack::downlink::tx_ack: Log downlink-frame for device device_eui=ac1f09fffe08e82c
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.774252Z DEBUG gRPC{uri=/api.DeviceService/Enqueue}: chirpstack::api: Started processing request
May 14 17:55:01 chirpstack[38585]: 2023-05-14T17:55:01.777160Z INFO gRPC{uri=/api.DeviceService/Enqueue}: chirpstack::api: Finished processing request status="200" latency=2.918379ms
I will be taking this topic to Datacake support as well.
For clarification, initially I opened this topic as I was not getting any response from Chirpstack server at all. This was fixed after correcting the nginx API proxy pass parameters > thanks to datnus