This sounds somehow like a duplicate of Keycloak Docker behind loadbalancer with https fails
Set the request headers X-Forwarded-For and X-Forwarded-Proto in nginx. Then you have to configure Keycloak (Wildfly, Undertow) to work together with the SSL terminating reverse proxy (aka load balancer). See http://www.keycloak.org/docs/latest/server_installation/index.html#_setting-up-a-load-balancer-or-proxy for a detailed description.
The point is that nginx is terminating SSL and is forwarding the requests to Keycloak as pure http. Therefore Keycloak/Wildfly must be told that the incoming http requests from nginx must be handled like they were https.