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
In general it's Microsoft.Bcl.Build failling. In my case was after a merge and Git add invalid data to my web.config. Just open the web.config file and check for lines like this:
Remove them.