I created this app in Web AppBuilder: ArcGIS Web Application
I want to host it on our server. Here were my steps:
1. I first created a local test environment through the IIS Manager called C:\inetpub\wwwroot\GISApi
2. Through AGOL I clicked Create --> App: Using Web AppBuilder
3. Using Web AppBuilder developer version I created the app. It has two widgets, neither of which require a subscription to use. The content of the app (CSV file, hosted feature layer, web app, and the app) are all shared publicly.
4. After saving the finished app I went back to AGOL to download the zipfile.
5. I copied the zip to the GISApi folder then extracted it, and renamed the folder "Cooling".
6. I then tried running the app a few ways:
a. I double clicked the index.html file, but the page never loaded:

b. I put these URLs in the browser (Chrome) https://localhost/gisapi/Cooling/index.html ,
http://www.willcogis.org/website2014/gis/Cooling
Here's what happened:

There's no need for a proxy because the app's shared publicly.
I then put a copy of the Cooling folder in our production folder. This is where the error message in the question title came from:

I contacted ESRI and the tech downloaded the zip to his local server. He had the app up and running with no problems. I never went into the code at all so I'm guessing it might be my PC? At the moment I have a colleague in another department attempting to run the app from his PC on a different server.