OAuth2 authentication problem

Hello *, I’m trying to implement OAuth2 authentication on a Phoenix app agains a Rails (doorkeeper) provider using ueberauth and a custom built adapter which right now is very very similar to the Github one.
I’m having some problem handling the redirect_uri, for some reasons I can’t understand it’s returning a “connection refused” but I have absolutely no idea who’s refusing what here. This “connection refused” is thrown by https://github.com/scrogson/oauth2/blob/master/lib/oauth2/client.ex#L248
If I understand OAuth correctly the provider should call the client with a code (and it happens) and this call is POSTed to the provider to get a token, that is the connection that gets refused. Problem is, I can’t see any request to the provider locally in my logs, I’m rather sure something breaks before, but hackney tracing gives me output that is apparently correct, but doesn’t end up calling the provider.

I have 0 ideas how to keep debugging this, and I’m very open to every suggestion.