Generic authentication responds with HTTP/500 on authorisation jupyterHub - oauth-2.0

Question

Generic authentication fails following oAuth2 authorisation, and produces an uncontextualised and unhandled HTTP/500 error.
Context
The hub and the authentication servers are run on separate Docker containers. They are both served through Nginx, which itself is housed on a a container that is independent of the two servers.
The hub runs on a subdomain, and all proxies are handled by Nginx.
The authorization is handled through the Spring oauth
The authentication server has been tested separately and is fully operational.
Hub server logs
[JupyterHub] [INFO] [302 GET /hub/oauth_login?next= -> http://localhost/oauth2/authorize/?redirect_uri=http%3A%2F%2Flab.localhost%2Fhub%2Foauth_callback&client_id=[secret]&response_type=code&state=[secret] (#XXX.XXX.XXX.XXX) 1.87ms]
[JupyterHub] [ERROR] [Uncaught exception GET /hub/oauth_callback?code=[secret]&state=[secret] (XXX.XXX.XXX.XXX)
HTTPServerRequest(protocol='http', host='lab.localhost', method='GET', uri='/hub/oauth_callback?code=[secret]&state=[secret]', version='HTTP/1.1', remote_ip='XXX.XXX.XXX.XXX')]
Traceback (most recent call last):
File "[...]/tornado/web.py", line 1543, in _execute
result = yield result
File "[...]/oauthenticator/oauth2.py", line 182, in get
user = yield self.login_user()
File "[...]/jupyterhub/handlers/base.py", line 473, in login_user
authenticated = await self.authenticate(data)
File "[...]/jupyterhub/auth.py", line 257, in get_authenticated_user
authenticated = await maybe_future(self.authenticate(handler, data))
File "[...]/oauthenticator/generic.py", line 116, in authenticate
resp = yield http_client.fetch(req)
tornado.curl_httpclient.CurlError: HTTP 599: Failed to connect to localhost port 80: Connection refused
[JupyterHub] [DEBUG] [No template for 500]
[JupyterHub] [ERROR] [{
"X-Forwarded-Host": "lab.localhost",
"X-Forwarded-Proto": "http",
"X-Forwarded-Port": "80",
"Cookie": "oauthenticator-state=[secret]:oauthenticator-state|120:[secret]"",
"Accept-Language": "en-GB,en-US;q=0.9,en;q=0.8",
"Accept-Encoding": "gzip, deflate, br",
"Referer": "http://localhost/oauth2/authorize/?redirect_uri=http%3A%2F%2Flab.localhost%2Fhub%2Foauth_callback&client_id=[secret]&response_type=code&state=[secret]",
"Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
"User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36 OPR/55.0.2994.37",
"Upgrade-Insecure-Requests": "1",
"Cache-Control": "max-age=0",
"Connection": "close",
"X-Nginx-Proxy": "true",
"X-Forwarded-For": "XXX.XXX.XXX.XXX,::XXXX:XXXX.XXXX.XXX.XXX",
"X-Real-Ip": "XXX.XXX.XXX.XXX.1",
"Host": "lab.localhost"
}]
[JupyterHub] [ERROR] [500 GET /hub/oauth_callback?code=[secret]&state=[secret] (#XXX.XXX.XXX.XXX)
The hub server never gets past the authorisation for it to start dealing with the token and user data. You see, this error occurs when the hub server tries to resolve the callback by the authentication server, not when requesting the token. The GET request is that which follows the authorisation in the auth server. That's what makes this quite bizarre.