Error: nuget configuration is invalid

Question

It's a project in Visual Studio 2015 actually, but there is no category of VS2015 in category selection so posting under VS2010.

 

Everything was working fine and suddenly I get this error while I try to add a new nuget package by right clicking on project and select "manage nuget packages...":

 

"<Project-Name> nuget configuration is invalid"

 

Any idea why this can happen?