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

I met this issue today and fix it by following:

If you have delete the feed from VSTS, then you need to delete it from VS\Tools\Options\Nuget Package Manager\Package Sources:

If you didn't delete the feed in VSTS and want to use it, sign in with your VSTS team project account from VS upper right corner and restart VS:

This is may be an addition to the actual answer. I had to disable the check boxes as shown in below image to get the issue resolved. I think it might help to someone.

I had the same issue and I fixed issue by disabling my custom NuGet servers. Go to Tools->Options->Nuget Package Manager and disable your custom NuGet servers