First some background. At the end of 2012 we migrated our vs2008 solution to vs2010 but we still target .NET 3.5. (I know nothing but the latest and greatest here!)
Hack, but solved it by copying:
c:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v10.0\WebApplications*.*
to
c:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v11.0\WebApplications*.*