Cannot connect to vm with Windows Remote Desktop Connection

[ndanger000]

I am trying to set up a virtual machine that can be accessed remotely, and have having problems accessing it with Windows Remote Desktop Connection.

I am running Virtualbox 5.2.18 on Debian 9 stable. I have created a virtual machine (Debian 9 guest). I have followed the instructions in the manual for creating the various SSL/TLS certificates and am using external authentication with VBoxSimpleAuth. I start the machine with

$ vboxheadless --startvm <machine_name> --vrde on

After doing so, I can connect to the vm with no problems using krdc on my Linux laptop (actually, a Linux guest running on a MacOS host laptop). But connecting with Windows Remote Desktop Connection fails. When I try, Remote Desktop Connection first complains about my self-signed certificates, so it seems to be connecting and communicating. After I say to accept the certificate anyway, I get an error that Remote Desktop Connection "can't connect to the remote computer." That's all I get.

Here are what I think to be the relevant lines from the virtual machine log:

00:32:05.708034 VRDP: Using TLS connection. Protocol: TLSv1.2
00:32:05.708049 VRDP: Server certificate: [...]
00:32:05.721145 VRDP: Methods 0x00000000
00:32:05.721158 VRDP: Channel: [cliprdr] [1004]. Accepted.
00:32:05.722804 VRDP: Client seems to be MSFT.
00:32:05.722817 VRDP: Logon: SLI100W-010 (aaa.bbb.ccc.ddd) build 2600. User: [] Domain: [] Screen: 0
00:32:05.723110 AUTH: User: []. Domain: []. Authentication type: [External]
00:32:05.723876 AUTH: external authentication module returned 'access denied'
00:32:05.723885 AUTH: Access denied.
00:32:05.723893 VRDP: Connection closed: 21

I never get a request for a username, and I get similar log messages when I provide a username in the RDC connection dialog.

Any ideas?