Import end devices fails with "An unknown error occurred. Please try again later."

Importing devices (for v2 → v3) fails with “an unkown error”. Any hints on what’s my mistake?
Die json seems to be ok as process log doesn’t complain.

The file format follows the End Device API

Process log:

Converting end device templates…
  "end_device": {
    "ids": {
      "device_id": "112-13",
      "application_ids": {
        "application_id": "tf-wst01"
      "dev_eui": "A01010101010101B",
      "join_eui": "7010101010101013"
    "created_at": "0001-01-01T00:00:00Z",
    "updated_at": "0001-01-01T00:00:00Z",
    "name": "112-13",
    "attributes": {
      "deepness": "11.09",
      "dev_id_knd": ""
    "lorawan_version": "MAC_V1_0_2",
    "lorawan_phy_version": "PHY_V1_0_2_REV_B",
    "frequency_plan_id": "EU_863_870_TTN",
    "supports_join": true,
    "root_keys": {
      "app_key": {
        "key": "B0101010101010101010101010101015"
    "mac_settings": {
      "supports_32_bit_f_cnt": true,
      "status_time_periodicity": "0s",
      "status_count_periodicity": 0
  "field_mask": {
    "paths": [
Creating end devices…

Import operation complete