When I create new ASP.NET 4.5 web forms application from vs2012 and update all nuget packages, I receive this error on build:
ASP.NET 4.5
vs2012
nuget
You have added a reference of a file which actually doesn't exists in your solution folder.
In my case issue have revealed after I merged a branch which broke my web.config file. It added comment character sequence <!-- without closing part -->.
web.config
<!--
-->
Making file markup valid fixed the issue.