NginX works with an index.php file, but not with the other

Hello Guys,

I have been given a task to migrate a very old website to a new server. The old server was using PHP 5.4, the new one is using PHP 5.6, but this is not the issue I think. My main problem is, if I set up the NginX config nicely the browser gives me an internal server error code 500. Not NginX, the browser doesn't even gets there and the logs do not log anything.
HOWEVER: If I change only the root path to somewhere inside the original root and I create a PHPInfo file as index.php, then that IS displayed in the browser without any errors.
How come the browser can see the "website" correctly if it is set to the phpinfo index.php, but can't even get to NginX if it points to the original index.php?

Here is the NginX conf file with some details outlined:
http://pastebin.com/3MJC79q3