Loading this assembly would produce a different grant set from other instances
I had this error when I installed .NET 4.5.1.
I fixed it by deleting the .suo files in my solution folder (you may have to show hidden files to see them).
Next time I started it up everything was back to normal.
I've been searching for weeks now and found the culprit.
In my case it was a new code signing certificate in the solution with the old one in my local certificate store.
When i installed the new certificate the error disappeared.