Hi Bruce,
thanks for using my plugin and your detailed report. I haven’t run the plugin with your configs but I guess the problem is the last point:
I am using Nginx Admin (v4.5) (http://nginxcp.com/) as a front end reverse proxy
Probably the request to the Amazon webservice from the plugin does not get through. I found many similar discussions on stackoverflow:
http://stackoverflow.com/search?q=502+Bad+Gateway+nginx+proxy
As I do not have much experiences with nginx and front end reverse proxy, I am sorry that I can not give you more information / a solution. But I am interested in finding the solution here of course. Please let me know when you are one step closer to resolving this issue.