The issue was a PHP parse error which I overlooked when I added a new file. The weird thing is, I had nothing in the logs (E_ALL, display_errors is off, but all logs are enabled and I tailed them using multitail) and nginx only displayed a blank page. The errors above were in nginx's own log file.

The multitail command lets you run the "tail" command on more than one file at a time and view them split out in your console for easier reference. The full error message reads:

client closed prematurely connection, so upstream connection is closed too while sending request to upstream, client: a.a.a.a, server: localhost, request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/tmp/.fastcgi.till/socket:", host: "localhost"