Problem to send downlink via MQTT clients

In the past I had no problem to send downlink messages via MQTT client. Yesterday, after many months, I tryed the same procedure but no downlink message was inserted in the downlink queue.
I am using MQTT BOX as client and I had no problem to connect and to receive messages from the subscribed topic.

The topic to publish is: v3/nps0000000000000001@ttn/devices/eui-/down/push where is the DEVEUI of the device that I would like to configure.

The Payload Type is: Strings / JSON / XML / Characters

The payload is:
{
“downlinks”: [{
“f_port”: 1,
“frm_payload”: “AAG7AASjCwEDrNg=” ,
“priority”: “NORMAL”
}]
}

after presing the PUBLISH button I obtain the following result:
Topic to publish
v3/nps0000000000000001@ttn/devices/eui-0004A30B0103ACD8/down/push
QoS
Retain
Payload Type
e.g: {‘hello’:‘world’}
Payload
{
“downlinks”: [{
“f_port”: 1,
“frm_payload”: “AAG7AASjCwEDrNg=” ,
“priority”: “NORMAL”
}]
}
Publish
{ “downlinks”: [{ “f_port”: 1, “frm_payload”: “AAG7AASjCwEDrNg=” , “priority”: “NORMAL” }] }
topic:v3/nps0000000000000001@ttn/devices/eui-0004A30B0103ACD8/down/push, qos:0, retain:false

but no downlink message is displayed in the Live Data section for the selected end device.

The downlink transmission is correctly working from the Messaging section.