Manage NugetPackage for Solution can't find any online packages

Somehow, the "Manage NuGet Packages for Solution..." Dialog doesn't show any online packages anymore (no items found.):

But the "Manage NuGet Packages..." dialog at the project level works fine.

I already tryed to start Visual Studio (2013 Update 4) with the /Log Parameter but cannot find any errors in the ActivityLog.xml.

Any Ideas?