OTAA denied using RN2483 + iC880A&RPi

We're trying to set up a new gateway in Varsseveld (Netherlands). The hardware used for the gateway is a RaspberryPi 3 with a ic880a concentrator board. I'm experience some problem when I try to connect a node (RN2483 dev kit from microchop) using Over the air activation (OTAA).

The gateway is configured using the new backend, I think the problem isn't here. We're using the following local_conf.json file.

{
  "gateway_conf": {
    "gateway_ID": "B827EBFFFECF29A7",
    "servers": [{
      "server_address": "router.eu.thethings.network",
      "serv_port_up": 1700,
      "serv_port_down": 1701,
      "serv_enabled": true
    }],
    "ref_latitude": 51.941649,
    "ref_longitude": 6.481048,
    "ref_altitude": 0,
    "contact_email": "b.knol@singa-bv.nl",
    "description": "ttn-ic880a"
  }
}

First I got my device unique identifier from the RN2483 module:

mac get deveui
0004A30B001B5FD7
sys get hweui
0004A30B001B5FD7

Then I created a application and registered a new device in it:

ttnctl.exe devices register 0004A30B001B5FD7
  INFO Generating random AppKey...
  INFO Registered device                        AppKey=2634943C13D15FE0A625F776A5F628FD DevEUI=0004A30B001B5FD7

Set all required parameters in the RN2384 module to perform a OTAA:

sys factoryRESET
RN2483 1.0.1 Dec 15 2015 09:38:09
mac set appkey 2634943C13D15FE0A625F776A5F628FD
ok
mac set appeui 70B3D57EDxxxxxxx
ok
mac save
ok

Join the network:

mac join otaa
ok
denied

The module attempted to join the network, but it was denied. Has anyone seen any issues like that? Or am I missing something? Any help is welcome!

My firmware version is ok according to this topic: http://forum.thethingsnetwork.org/t/otaa-with-rn2483-using-new-backend/1986

Thanks in advance.

Bart