Windows Defender trying to scan network files, failing with Security Event ID 4625

I have a Hyper-V host and guest both running Server 2008 R2. The host is stand-alone (not domain-joined); the guest is a domain controller.

I recently changed the guest to do a Full Scan instead of just a Quick Scan.

Now I get over 1000 security errors on the _host_ at the same time that the _guest_ is completing its scan. The errors indicate that the guest's machine account is trying and failing to connect to the host.

I had the same problem with Security Essentials last year, as documented here:

http://answers.microsoft.com/en-us/protect/forum/mse-protect_start/getting-4625-security-errors-on-server-after/628c5c6b-8bc7-4af5-ac69-aca114b821ac

I was able to solve that by adding the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft Antimalware\Scan\DisableScanningNetworkFiles = 1

I've tried what I thought would be the analogous key for Defender:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Defender\Scan\DisableScanningNetworkFiles = 1

but the 4625 errors continue.

How can I stop Windows Defender from scanning network files on a Full Scan?