Region change (network layer settings) for device gives invalid field error

TTN Network and Services
1

We have some devices that we already registered on TTN as EU868 devices, but during production we decided that we would make some of them for the AU915 band. Now I would like to change the network layer settings for these devices in TTN, but it gives me the following error:

{
  "code": 3,
  "message": "error:pkg/networkserver:field_value (invalid value of field `session.dev_addr`)",
  "details": [
    {
      "@type": "type.googleapis.com/ttn.lorawan.v3.ErrorDetails",
      "namespace": "pkg/networkserver",
      "name": "field_value",
      "message_format": "invalid value of field `{field}`",
      "attributes": {
        "field": "session.dev_addr"
      },
      "correlation_id": "382484e918734f3d93fb68c10c088572",
      "code": 3
    }
  ]
}

Since it complains about a session.dev_addr I also did a reset of the MAC state before saving; this gives me the next error:

{
  "code": 13,
  "message": "error:pkg/rpcserver:rpc_recovered (Internal Server Error)",
  "details": [
    {
      "@type": "type.googleapis.com/ttn.lorawan.v3.ErrorDetails",
      "namespace": "pkg/rpcserver",
      "name": "rpc_recovered",
      "message_format": "Internal Server Error",
      "correlation_id": "020363ee3dc3447ea50b41989565015b",
      "code": 13
    }
  ]
}