Build Fails After Changing Configuration to Release

I would like to start using the web.config transform feature in Visual Studio when publishing to our test and production site. However, the release build fails while the debug build works fine.

I've believe I'm made the build settings the same. Although the only differences were 'Enable DEBUG constant', 'Enable optimizations', and 'Generate debug info'.

Anyway, the build error is "Could not load file or assembly 'file:...dll' or one of it's dependencies. Operation is not supported. (Exception from HRESULT: 0x80131515).

The mentioned dll was from the Ajax Control Toolkit. I used Nuget to 'install' the Ajax Control Tookit package instead of just including a reference to the dll. Then I got the same error for Microsoft's AntiXSS. So I again used Nuget to 'install' AntiXSS. Unfortunately, now I'm getting this error on another third party API that doesn't have a Nuget installer. Also, I'm getting this error on a dll I've created that I include in a number of projects here.

Does someone know what is causing this error? What should I do to fix this error? Do I need to create a Nuget package?

Thanks