Problem proxying NuGet packages hosted by GitHub Packages

• ,create a new NuGet (proxy) repository
  • name it "agents-net"
  • use https://nuget.pkg.github.com/agents-net/index.json as Remote storage URL
  • configure authentication with GitHub username / personal access token

=> repo is available at http://SERVERURL/repository/agents-net/

• add NuGet package source in Visual Studio
  • http://SERVERURL/repository/agents-net/index.json
• create new project in Visual Studio or use an existing one
  • Manage NuGet Packages for the project
  • select new package source
  • select browse tab
    • check "include prerelease"

 => VS shows error

• in Output / Package Manager the following error is shown
  "[Agents.Net] Failed to retrieve metadata from source 'http://.../repository/agents-net/v3/query/1?q=&skip=0&take=26&prerelease=true&semVerLevel=2.0.0'.
  Response status code does not indicate success: 500 (javax.servlet.ServletException: java.lang.NullPointerException)."
• Log View shows stacktrace to NullPointerException (see also attached log file)