It could be that the path to your solution is too long:
Meaning:
C:\MyProject\Folder\SubFolder…
should be Under “256 Character”.
http://wcfvs.blogspot.com/2011/04/could-not-resolve-mscorlib-for-target.html
I found this thread when I had the same issue but had no reference issues. I found the link above and it solved my problem. Hopefully this will help someone else who comes here with the same problem.