VirtualBox Remote Instance

Hi.

Here's my set-up:
~:$ freebsd-version -ru
12.2-RELEASE-p7
12.2-RELEASE-p11

~:$ pkg info | awk '/virtualbox/ {print $1}'
virtualbox-ose-6.1.26_3
virtualbox-ose-additions-6.1.26_1
virtualbox-ose-kmod-6.1.26

Here's the problem:
When running VirtualBox over SSH with X11-forwarding, I'm unable to start windowed virtual machines from the GUI. A window comes up saying "Creating process for virtual machine '...' (GUI/Qt) ... (1/2)" and nothing happens.
- VMs start fine using the "Headless Start" menu option.
- VMs fail to start using "Normal" or "Detachable" menu options.
- VMs start fine using any of the three options locally (on the VMs host).

Here's the kicker: 
If I'm logged in on the VM's host, both "Normal" and "Detachable" work over SSH. What is causing that, and how can I fix it so I don't have to be logged in locally on the host to start windowed instances remotely?

More details:
I've enable the following ExtraData items on the VM, but nothing comes up while waiting for the failed windowed session to start:
setextradata burp GUI/Dbg/Enabled true
setextradata burp GUI/Dbg/AutoShow true

I've also tried running the windowed VM instance directly without result:
~:$ ssh burp.host "virtualboxvm --startvm burp --debug-command-line"

Furthermore, log-outs hang on the VM's host if a windowed instance of the VM is running over SSH.

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