WordPress deploy results with a 304 error and redirecting to :8888

If your load balancer will let you do this, look for the full admin path: "www.domain.com/wp-admin/" in the url and send requests for that to the primary server.

While your at it you should change the wp-admin folder to some other random name for security.

This doesn't directly answer your question, but is probably a better option you may not have considered. Instead of using a plugin to do caching, using Nginx page caching. It's much faster because you don't have to make a call to PHP, which eliminates a lot of overhead.

The downside is it's tricky to invalidate the Nginx cache unless you pay for the commercial version of Nginx. You can build Nginx with plugins that do the job, but the Wordpress / Nginx caching integration is not great. None I've found work well. So you need to set your cache maximum life carefully. Interestingly, on a busy site caching for even a few seconds can have benefits. My sites change rarely, and if I need to I can just rm -rf the correct directories where the nginx page cache lives - which is actually in memory.

I have a tutorial on this here, and there will be many others around. There's a great article on Nginx microcaching here.

SF likes actual data on the page in case websites disappear.

In your nginx.conf

fastcgi_cache_key "$scheme$request_method$host$request_uri";

At the top of your site file, or in your nginx config

fastcgi_cache_path /dev/shm/nginxcache levels=1:2 keys_zone=CACHENAME:10m inactive=1440m; # Centos / Amazon Linux in RAM, 1440 minutes = 24 hours

In your location block that calls PHP

fastcgi_pass php56-fpm;
include        fastcgi_params;
fastcgi_param  SCRIPT_FILENAME $document_root$fastcgi_script_name;

fastcgi_cache_use_stale error timeout invalid_header http_500;
fastcgi_ignore_headers Cache-Control Expires Set-Cookie;
fastcgi_cache CACHENAME;
fastcgi_cache_valid 200 1440m;
fastcgi_cache_valid 403 404 405 410 414 301 302 307 60m;
add_header X-Cache $upstream_cache_status; # This can be removed if desired

fastcgi_cache_methods GET HEAD;
fastcgi_keep_conn on;

That tutorial I linked to has a lot more information and explanation.