MQTT bridging: error connecting to remote broker

Configuration
1

I’m running Mosquitto server with no problem, now I would want to bridge the server to shiftr.io, so I started to use the external configuration as below:

{
  "logins": [
    {
      "username": "***",
      "password": "***"
    }
  ],
  "anonymous": false,
  "customize": {
    "active": true,
    "folder": "mosquitto"
  },
  "certfile": "***.crt",
  "keyfile": "private.key",
  "require_certificate": false
}

But I get the error when connecting to shiftr.io

1573747520: Connecting bridge shiftr_io (broker.shiftr.io:1883)
1573747520: Socket error on client local.core-mosquitto.local.hass.io.shiftr_io, disconnecting.

the connection to local mosquitto broker is up as usual.

Tried to connect to shiftr.io with node-red that is running on the same docker container as mosquitto and the connection to the remote broker work like a charm.
So, what’s the problem with the bridging?
The moquitto configuration is the following:

connection shiftr_io
address broker.shiftr.io:1883
topic # out 0
remote_username ***
remote_password ***
2

This is the extended logging of the mosquitto broker:

1573749218: Bridge local.core-mosquitto.local.hass.io.shiftr_io doing local SUBSCRIBE on topic #
1573749218: Connecting bridge shiftr_io (broker.shiftr.io:1883)
1573749218: Bridge core-mosquitto.local.hass.io.shiftr_io sending CONNECT
1573749218: Socket error on client local.core-mosquitto.local.hass.io.shiftr_io, disconnecting.