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 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 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 happening 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 request fails with the error below.
Resource not found for the segment 'Packages'
Also NuGet produces the strange output on NuGet API calls that issues at VS search packages stage, please see here: http://pastebin.com/qveadrZB.