Performing a fresh install of ezplatform with the legacy bridge I came across a frustrating issue where PHP-FPM is not redirected to app.php causing Nginx to simply display "Input file not specified". In the nginx error log is shows;
FastCGI sent in stderr: "PHP message: PHP Warning: Unknown: failed to open stream: Success in Unknown on line 0
Unable to open primary script: /var/www/mysite/web (Success)" while reading response header from upstream, client: x.x.x.x, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "mysite"

The solution for me was to comment out "cgi.fix_pathinfo=1" in /etc/php5/fpm/php.ini

Robin on slack helped me find this solution and also noted this article as relevant http://serverfault.com/questions/627903/is-the-php-option-cgi-fix-pathinfo-really-dangerous-with-nginx-php-fpm

This setting does not effect eZ 5.5 and it was previously recommended to be set. So as people upgrade to eZ Platform this is going to bit them and cause a lot of frustration to track down. Could we document this somewhere perhaps?

Note this issue looks identical to https://jira.ez.no/browse/EZP-25336