It's optional but adds more prevention for the issue you encounter.
And please change your nginx code to this:
Code:
### FastCGI Cache ################
map $http_cookie $nocachecookie {
default 0;
~xf_fbUid 1;
~xf_user 1;
~xf_logged_in 1;
}
map $request_uri $nocacheuri {
default 0;
~^/register 1;
~^/login 1;
~^/validate-field 1;
~^/captcha 1;
~^/lost-password 1;
~^/two-step 1;
}
fastcgi_cache_path /tmp/nginx_fastcgi_cache levels=1:2 keys_zone=fastcgicache:200m inactive=30m;
fastcgi_cache_key $scheme$request_method$host$request_uri;
fastcgi_cache_lock on;
fastcgi_cache_use_stale error timeout invalid_header updating http_500;
fastcgi_ignore_headers Cache-Control Expires Set-Cookie;
### FastCGI Cache ################