VS2015 C# Localhost ASHX Error (But it works on Server?) HRESULT 0x80131401

I'm building a website in VS2015 (C# using Razor) I see many posts where people say that they receive this error on their server, but not localhost - my issue is the opposite. I'm seeing this error in every *.ashx file preamble in this specific Website only while running Localhost on my development machine:

ASP.Net runtime error: Loading this assembly would produce a different grant set from other instances (Exception from: HRESULT 0x80131401)

This error prevents me from using Intellisense in the file - and I really need it for this next handler.

What I've Determined: - The issue is specific to this Website. I have other websites that do not produce this error, so there isn't a need to modify registry or change PC configurations.

What I've tried:

• IISReset (Restart PC) [*before I realized problem was local]
• Changing Registry by adding LoadingOptimizer DWORD [*before I realized problem was local]
• Compared XML of a working Web.config file. (Other than assembly differences I played around with other differences and still nothing)

Any suggestions?