I also had this error some time ago.
This helped for me:
1. Delete the references with an exclamation mark and readd them. (Only if you have some)
2. Set the target framework for all applications in the project to .Net 4.0 or 4.5
Read no one's advice but Fab's.
Originally Posted by Organized Chaos
What source are you using?
Club's one
Originally Posted by ossimc82
I also had this error some time ago.
This helped for me:
1. Delete the references with an exclamation mark and readd them. (Only if you have some)
2. Set the target framework for all applications in the project to .Net 4.0 or 4.5