ASP.NET MVC5 OWIN Facebook authentication suddenly not working

Question

Update 2017!

The issue I had when I posted the original question has got nothing to do with the recent changes Facebook made when they forced everyo

Answer 1

This drove me insane. All was working until I deployed to my staging environment. I was using Microsoft.Owin.Security.Facebook version 3.0.1 from Nuget. Updated it to the prelease version 3.1.0 from Nuget and I no longer got the access denied error...