Nuget Package - feed (VSTS) :Exception 'System.AggregateException' thrown when trying to add source

I have created a new feed with in Package Release hub (VSTS), installed the credentials, then added the package source.

Now, I am using Visual Studio 2015 to install Micrososft.Aspnet.mvc to a project, however it gives the following error:

Exception 'System.AggregateException' thrown when trying to add source
'https://mysite.pkgs.visualstudio.com/DefaultCollection/_packaging/MyLogUtils/nuget/v3/index.json'.
Please verify all your online package sources are available.    

I need to install NuGet packages normally, so I removed the feed from VSTS. However, the problem persists. How can this problem be resolved?