Credentials problem from build server

Hi,
I'm trying to get our build server to restore packages from proget during build. This is being run as the user SVC_TFS, which is a specific system account for uploading and downloading packages during auto build. I've granted the user Feeds_AddPackage, Feeds_DownloadPackage privileges in the proget admin panel (we are using AD integration, so this is a domain account).

I've added those credentials to the nuget.config file, which looks like this:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="http://go.microsoft.com/fwlink/?LinkID=206669" value="http://go.microsoft.com/fwlink/?LinkID=206669" />
    <add key="https://www.nuget.org/api/v2/" value="https://www.nuget.org/api/v2/" />
    <add key="internal" value="http://the-proget-server:777/nuget/packages" />
  </packageSources>
  <disabledPackageSources />
  <packageSourceCredentials>
    <internal>
      <add key="Username" value="DOMAIN\SVC_TFS" />
      <add key="ClearTextPassword" value="Password" />
    </internal>
  </packageSourceCredentials>
</configuration>

However the msbuild restore command reports an error. It's executing this command:

""C:\Builds\4\Code\Our.Project\src\.nuget\NuGet.exe" install "C:\Builds\4\Code\Our.Project\src\Our.Project.Tests\packages.config" -source ""  -NonInteractive -RequireConsent -solutionDir "C:\Builds\4\Code\Our.Project\src\ "

and outputting:

Using credentials from config. UserName: DOMAIN\SVC_TFS
Please provide credentials for: http://the-proget-server:777/nuget/packages

Any ideas on what why it would be asking for credentials? Is there any way I can stop it doing that?

Product: ProGet
Version: 3.2.0

This error is coming from nuget.exe; depending on the version of nuget.exe you're using, AD / integrated authentication simply will not work.

It's supposedly fixed in 2.8.2, but some users say it doesn't work in 2.8.3. It's pretty broken in 2.8.1 and 2.8.0. Although it sometimes works in 2.7. The 2.6's seem to be fine, for the most part.

ProGet.exe, from the now-defunct client tools, of course will work fine, or a simple HTTP PUT operation.