Seems in .NET 4.5 windows can block loading plugins due to coming from an unknown source (bleugh)
I was having this same issue. The reason was that the Assembly file was blocked by Windows. I resolved it by right clicking on the Assembly file and selecting properties. In the Properties dialog, click Unblock button under the General tab and click Apply and then OK.
That's apparently enough to fix it, but we should handle (and surface) the error better, and potentially see if we've got another way around it