I have recently upgraded from VS2012 RC to RTM, and since the upgrade to RTM, I can no longer access nuget. I receive the following error in the Manage NuGet Packages window.
I started getting this in VS2015 on one of my machines. I had an additional package feed on myget which constantly asked me for credentials and failed even when entering correct credentials. What helped me was clearing the nuget cache and config by deleting these two nuget folders:
After that I restarted Visual Studio and added my custom package source again.