Visual Studio - Nuget - Unable to load the service index for source

This is my NuGet.config :

<?xml version="1.0" encoding="utf-8"?><configuration><packageSources><add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" /></packageSources><disabledPackageSources><add key="Microsoft and .NET" value="true" /></disabledPackageSources></configuration>

Now, in Visual Studio, when I try to install 'geckofx45' package, I get the following error:

install-package geckofx45install-package : Unable to load the service index for source https://api.nuget.org/v3/index.json.At line:1 char:1+ install-package geckofx45+ ~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo : NotSpecified: (:) [Install-Package], Exception+ FullyQualifiedErrorId : NuGetCmdletUnhandledException,NuGet.PackageManagement.PowerShellCmdlets.InstallPackageCommand

Do you have any solutions?