VBox ignores vrde-auth-type

Packer
1

Hi all,
I’m provisioning a VM to test and ISO I’m building daily with following template:

  "builders": [
    {
      "guest_os_type": "RedHat_64",
      "boot_wait": "10s",
      "disk_size": "{{user `disk_size_vbox`}}",
      "headless": "{{user `headless`}}",
      "iso_checksum": "{{user `iso_checksum`}}",
      "iso_url": "{{user `iso_url`}}",
      "iso_interface": "sata",
      "output_directory": "output-{{build_type}}",
      "shutdown_command": "shutdown -P now",
      "ssh_password": "{{user `new_ssh_pass`}}",
      "ssh_timeout": "{{user `ssh_timeout`}}",
      "ssh_username": "root",
      "type": "virtualbox-iso",
      "memory": "{{user `memory`}}",
      "vm_name": "{{user `iso_name`}}",
      "cpus": "{{user `cpus`}}",
      "firmware": "{{user `firmware`}}",
      "skip_export": "true",
      "vboxmanage": [
        ["modifyvm", "{{.Name}}", "--vrde-auth-type", "null"]
      ]
    }
  ],
  "provisioners": [
    {
      "type": "ansible-local",
      "playbook_file": "myplaybook.yml",
    }
  ],
  "variables": {
    "cpus": "2",
    "disk_size_vbox": "60000",
    "headless": "true",
    "iso_checksum": "none",
    "iso_url": "",
    "memory": "4096",
    "ssh_timeout": "30m",
    "firmware": "efi",
    "iso_build_path": ""
  }
}

everything works fine, but I need RDP connection to debug ISO installation process in case of issues.
I installed vbox extension packs, it binds the port properly but when I try to connect with Windows RDP client I got following error:

The connection cannot proceed because authentication is not enabled and the remote computer requires authentication be enabled to connect.

I tried to pass the option --vrde-auth-type null in VirtualBox manual https://www.virtualbox.org/manual/ch07.html and also in command help, the command seems to be executed properly:

Executing VBoxManage: []string{"modifyvm", "packer-virtualbox-iso-1747403398", "--vrde-auth-type", "null"}

but client still complains about authentication.

Can you help me understanding what I’m doing wrong?

I’m running VirtualBox 7.1 and packer 1.12.0-1