Error: FindPackagesById: EntityFramework.Core Response from https://www.postsharp.net="" nuget="" packages="" findpackagesbyid()?id="EntityFramework.Core" is not a valid NuGet v2 service response.
Now, an important note here: I'm on a machine that's seen various updates and changes to VS 2015, and this was a version of PostSharp that wasn't originally built for the RTM version of Visual Studio. So…this may be entirely circumstantial, but it's what I ran into.
And it wasn't just on that one package (others would give the same result) and it wasn't just on one project. I tried to isolate this, but couldn't find the source. Why was PostSharp getting in the way of my package restore? Even using DNU from the command line, after I explicitly uninstalled it? I started setting compiler variables to block PostSharp on those projects, but that got frustrating quickly, so I resorted to uninstalling everything I could find of it.
Unchecking that box above does the trick.
Might be an edge case if you run into this, but if you do, and this helps, consider buying Don a scotch!