nginx 401 error

[keith6014]

Using jupterhub 2.2.1
When using "jhub_remote_user_authenticator" via proxy (nginx) I keep getting
http://server/hub/login?next=%2Fhub%2F

ERR_INVALID_RESPONSE.

My proxy is definately getting the remote_user header. I confirmed in my access log. I also have underscores_in_headers on.

When I change it back to dummy authentication everything is working -- so I know my proxy settings are OK (atleast the binding).
Is there anything else I should be looking at?

Ideally, it would be great to have remote_user + PAM authentication working.