Enabled loadFromRemoteSources as the plugins weren’t being picked up anymore because of a change in the .Net Framework.

From MSDN:

In the .NET Framework version 3.5 and earlier versions, if you loaded an assembly from a remote location, the assembly would run partially trusted with a grant set that depended on the zone in which it was loaded. […] If you try to run that assembly in the .NET Framework version 4 and later versions, an exception is thrown; you must either explicitly create a sandbox for the assembly, or run it in full trust.

So I added the following to app.config to get it working again:

    <loadFromRemoteSources enabled="true" />