Nuget restore fails on Azure Devops with message "unable to load the service index for source"

I found a solution - add the following package source to %appdata%\NuGet\nuget.config:

<add key="Microsoft Visual Studio Offline Packages" value="C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\" />

Complete file looks like this:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="Microsoft Visual Studio Offline Packages" value="C:\Program Files (x86)\Microsoft SDKs\NuGetPackages\" />
    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
    <add key="MyFeed" value="https://xxxxxxxxxx.pkgs.visualstudio.com/_packaging/xxxxxxxxxx/nuget/v3/index.json" />
  </packageSources>
  <packageSourceCredentials>
    <MyFeed>
      <add key="Username" value="LocalBuildAgent" />
      <add key="ClearTextPassword" value="xxxxxxxxxxx" />
    </MyFeed>
  </packageSourceCredentials>
</configuration>

Also, check Regression in .NET SDK 500: 'dotnet tool install' fails with 401 (Unauthorized) when there is a private feed in NuGet.config #7524. This problem seems to be caused by .NET SDK 2.1.500.

Another workaround would be to uninstall that version:

The issue is not present in .NET Core SDK 2.1.400, e.g. it goes away when .NET Core SDK 2.1.500 is uninstalled, and reappears when SDK 2.1.500 is installed again.

I had to change the nuget installer to 4.8.1 in order for this to work after switching VSTS url to the new Azure Devops url.

Using the latest "Use .NET Core sdk 2.1.504" task worked for me. Seems there are some buggy versions of .NET Core sdk 2.1.5xx out there.