How it's possible that NuGet's Install-Package fails with Unable to find version 'x' of package 'y' when that exact version is released as NuGet to the official repository and it shown on the https://www.nuget.org/packages/y page.
Here is specific sample with nuget.org content and NuGet Install-Package command output.
UPDATE. Package sources are set to defaults (from nuget.org). Actually, you can see the issue just by running Install-Package Log4Net.Async -Version 1.3.0.
UPDATE 2. I want to highlight that it's not about some developer's machine - the same things happen on Visual Studio Online Hosted Build Controller.
UPDATE 3. Here is how NuGet tries to load package and it looks like NuGet API just broken, because all these requests fail with the error below.
Resource not found for the segment 'Packages'
Also NuGet produces a strange output on NuGet API calls that issued at VS search packages stage, please see here: http://pastebin.com/qveadrZB.