RDP connections fail with VRDP external auth types

[libIT]

Host: Ubuntu 18.04.3 LTS (GNU/Linux 4.15.0-55-generic x86_64)
Guest: parted magic live CD, bridged adapter

I'm having trouble with VRDP auth in VirtualBox 6.0.1. When VRDP auth type is set to null, RDP connections can be made successfully. When VRDP auth type is set to external, connections are negotiated, but end with the error message TCP Close; no successful RDP session occurs. I've tried both using both external auth methods, VBoxAuth and VBoxAuth simple. I've used multiple RDP clients. What am I missing? Thanks!

VBoxManage --version

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6.0.10r132072

VBoxManage list extpacks

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Extension Packs: 1
Pack no. 0:   Oracle VM VirtualBox Extension Pack
Version:      6.0.10
Revision:     132072
Edition:      
Description:  USB 2.0 and USB 3.0 Host Controller, Host Webcam, VirtualBox RDP, PXE ROM, Disk Encryption, NVMe.
VRDE Module:  VBoxVRDP
Usable:       true 
Why unusable: 

VBoxManage showvminfo pmagic

pmagic.txt

(6.61 KiB) Downloaded 22 times

VBox.log for pmagic vm:

VBox.log

(107.94 KiB) Downloaded 20 times

[socratis]

Just so that you know you're not alone, I've never been able to successfully RDP to a VM that used anything except "Null" authentication method. I don't know what's going on either...