Invalid fixed paths set in application downlink,error code 3

hey everyone

I have 2 lorawan gw(named A and B),using AS923-2-NDT(AS923-2 freq plan has test,fail…)
i have build a multicast device
then I use mqtt client login and send multicast msg ,if the gateway_id is A’s ID ,work fine ,multicast msg can downlink from gw A
but when i change to B’s ID , I got this err

{
  "end_device_ids": {
    "device_id": "multicast-as923-2",
    "application_ids": {
      "application_id": "jack-au915"
    },
    "dev_addr": "FFFFFFFF"
  },
  "correlation_ids": [
    "as:conn:01H3VGE4NEBT5YDV648Y5DFMZ8",
    "as:downlink:01H3VJ0PPGSBSCX15C7QR3HV11",
    "as:up:01H3VJ0PX7XVVAQQDZFF67ZK8J",
    "rpc:/ttn.lorawan.v3.NsAs/HandleUplink:01H3VJ0PX6Q824V0JRXTFMEBAS"
  ],
  "received_at": "2023-06-26T09:54:32.998693669Z",
  "downlink_failed": {
    "downlink": {
      "f_port": 10,
      "f_cnt": 4,
      "frm_payload": "qqEQ//////////8CAQEQ8A==",
      "class_b_c": {
        "gateways": [
          {
            "gateway_ids": {
              "gateway_id": "eui-ffff2c6a6f960016"
            }
          }
        ]
      },
      "priority": "NORMAL",
      "correlation_ids": [
        "as:conn:01H3VGE4NEBT5YDV648Y5DFMZ8",
        "as:downlink:01H3VJ0PPGSBSCX15C7QR3HV11"
      ]
    },
    "error": {
      "namespace": "pkg/networkserver",
      "name": "fixed_paths",
      "message_format": "invalid fixed paths set in application downlink",
      "code": 3
    }
  }
}

I dont kown why
by the way ,the gw A and B all working fine and online

question 2:
when i using ttn-lw-cli cmd on linux ubuntu
i dont kown how to assign the gw id to downlink push msg with cmd parame --class-b-c.gateways

i check a lot of TTN document page,failed to get the usage help msg
this is my cmd

ttn-lw-cli end-devices downlink push  jack-au915 multicast-as923-2 --class-b-c.gateways eui-ac1f09fffe0166b7-2 --f-port 10 --priority NORMAL --frm-payload 112233445566 --log.level debug

the eui-ac1f09fffe0166b7-2 is my gw id
i got this error

DEBUG	Connected to gRPC server	{"duration": 65.8767, "target": "au1.cloud.thethings.network:8884"}
DEBUG	Finished unary call	{"duration": 0.3738, "error": "error:pkg/networkserver:no_downlink_path (no downlink path available)", "error_correlation_id": "7270976304a74482a1ae14732e238723", "error_name": "no_downlink_path", "error_namespace": "pkg/networkserver", "grpc.method": "DownlinkQueuePush", "grpc.service": "ttn.lorawan.v3.AppAs", "grpc_code": "NotFound", "namespace": "grpc", "request_id": "c5f3cad1-40a6-41f2-9c79-11472987802d"}
error:pkg/networkserver:no_downlink_path (no downlink path available)
    correlation_id=7270976304a74482a1ae14732e238723

anyone help ?
thanks a lot

hey everyone
maybe i got the reason, i have configure gateway keep alive interval to 800s
it is too big , then i change to 5s
maybe fixed!

but i got another problem,
Whether it is unicast downstream or multicast downstream , the end device can receives message succ ,but in the live tab page shows failed , the result is “UNKOWN error”

i dont kown why

Please use the default 10 seconds for the interval.

OK I will do that
5s is ttn document suggest value

by the way , do you have any idea of my problem 2, multicast downlink msg and unicast downlink msg succ but ttn live data tab page tell me got a unkown error

why ?

No, no clue.