I ran into an issue like this before. With me it turned out to be some kind of issue with MonoDevelop.
I went here [MonoDevelop][1] and selected Packages, then downloaded and installed the .NetFramework 4.0 and GTK# for .net2.12.22. That seemed to fixed it. Might give that a shot. Be sure and close out of Unity while you install this stuff to be safe
[1]: Redirecting…