Oops. Sorry, I missed. The real problem is that with suggested configuration (i.e. always return 503 at server level) there is no way to reach /system/maintenance.html file for nginx.

So, it tries to get /system/maintenance.html for error body, and gets yet another 503. So it has to return hardcoded content.

The only solution is to allow nginx to access /system/maintenance.html somehow. Something like this:

error_page 503 /system/maintenance.html;

location / { if (-f ...) { return 503; } }

location /system/maintenance.html { # allow requests here - do not return 503 }

The if{} block should be in all locations where access should be disallowed, but not for /system/maintenance.html itself.