How to diagnose Resharper Unit Test Runner "Unable to load one or more of the requested types" error

When I run or debug my unit tests using Resharper Unit Test Runner, I get a dialog popping up that says "Unit Test Runner failed to run tests - Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information":

Now I have tried rebuilds, cleans, manually deleting folders, visual studio restarts, hardware restarts, looking in output/debug windows, and evening enabling R# "internal" mode so that I can see it's logs (written to %Temp%\JetLogs as I understand), but none of that resolves it or gives any clues at all. I have tried "debugging" R# but again the dialog pops up before the debugger hits any exceptions.

How the hell am I supposed to resolve this? It's extremely annoying!

I'm using:

• R# 2016.1.2
• NUnit 3.2.1
• Visual Studio 2015 Update 2 (14.0.25123)