could you please open the Dev Tools in your Browser (F12 or Cmd+Alt+I on Mac) and check the Network tab. You should see a failed request, could you somehow screenshot or copy information about that request, and also about the URL, that you are using to access Kibana in your Browser.
Thanks for that information. Are you sure the Kibana server is still up and running? So if you do a refresh without cache (Shift + F5) your Kibana instance will still load?
If that works: you are sure, that you have a proper connection to the required URL: http://192.168.51.59/api/console/proxy and no firewall or reverse proxy is blocking it? Also maybe try incognito mode of your browser to check, that it's not a browser plugin, that intercept these calls.
Okay, that request really shows, that you are running a reverse proxy (nginx) in front of the Kibana instance. Since _template is an Elasticsearch API and not a Kibana API, might both be running behind the same reverse proxy? Could you perhaps post the configuration of your nginx, maybe that helps troubleshoot the error.
I think the interesting part of the nginx config would be some location and proxy_pass directives in there. I assume that there is some mismatch in what path Kibana would need to be forwarded and what pathes are actually forwarded. But without the complete nginx config (and not just the first 30 lines) I can't give any more detailed explanation, sorry.
I unfortunately doubt, that this is the full config, since it's an invalid config and the nginx wouldn't start like that - it has an open http block in the end, and I highly doubt, that the config ends on a comment about GZIP setting.
I think we cannot help much more with the apparent network issue, over the forums, sorry
If you have a support subscription I would kindly ask you to open a support case for that, since they will be able to help you in way more detail, than we can help here any further.
Sorry, for that. Network issues - and I still think that's one - are always hard to debug without access to actual systems.