Well, it's worth considering patterns and anti-patterns in that question - complexity or length do not equate quality. The question has a few anti-patterns in it in my opinion.
I'm having The Most Bizarre Tech Problem. While my site works fine in development, and used to work fine in production, during the last build it has inexplicably stopped recognising the static assets, and is serving an old version of the favicon into the bargain. I've even tried recloning the whole repo, but I'm getting exactly the same problem. The old favicon doesn't even exist in the repo anymore.
...isn't a great description of the problem to someone not looking at it.
I have an nginx reverse proxy in front of the Cowboy server process, with the following config:
That might be useful, but a config dump on its own is a bit of an anti-pattern for a good question. If it worked before, what's changed is the important part of troubleshooting.
Updates are probably useful, if they are seen, and it's better to have them up front as things you have tried so far.
The source (at least for my main server, there seemed to be different problems causing the same error on the other one, which is weird but meh) of the problem ended up being something gone awry with the SSL certs which somehow resulted in access being blocked to only some of the content (???).
The answer doesn't really reflect a solution, just a workaround. Once again, considering that a cert is a cert, there's insufficient resolution to the problem.