I really guess that you have a glitch in your configuration.
The Graylog in your Browser tries to connect to the configured Graylog (http_external_uri) but the Graylog server itself tries to communicate to itself (http_publish_uri) so I guess that something is wrong with that.
Or you have a self signed certificate and did not add that to the jvm keystore for Graylog so Graylog can’t verify the certificate it uses.