Solved – System.ArgumentNullException: Value cannot be null.Parameter name: SandboxAppDomainHelper.Execute: The plug-in type could not be found in the plug-in assembly in Dynamics 365

We got the below error while in one of our plugin assemblies. 

As it turned out we were referencing the latest, version 9 Dynamics 365 assemblies in our plugin project, against our Dynamics 365 Environment which was still in version 8.2.

Adding the appropriate NuGet Package (i.e. 8.2.0.0) fixed the issue for us.

The helpful post

http://blogs.it.ox.ac.uk/benwalker/2017/09/12/system-io-fileloadexception-error-in-plugin-deployed-by-crm-developer-toolkit-to-dynamics-2015/

Hope it helps..