The problem is that the CLR v4 security policy model has changed since previous versions and are not sandboxing assemblies as before.
Adding the snippet below to an app.config file will allow your application to load dlls from remote systems:
<LOADFROMREMOTESOURCES enabled="true" />
You may also be able to fix the error by finding the assembly DLL in Windows Explorer, right clicking, choosing Properties, and then pressing the "unblock" button. The DLL has a stream that is marking it as an external file - and by clicking unblock you remove that designation.