This question has been answered by stefany. See the first response.
error_reporting = E_ALL display_errors = On
Did you try to enable errors in .htaccess? That will often throw a 500 error.
What did you put in php.ini for this?
I have this:
error_reporting = E_ALL display_errors = On
BTW, if the error occurs in a processor, you won't ever see it unless you look on the Network tag in Chrome. Dev. tools, or the Firefox equivalent (Ctrl-shift-i). You can click on a request and see the error (in ugly HTML) on the response tab.
tail -f -n 10 nginx_error.log
Ah. Good old nginx. I'm glad you got it sorted.
Do fatal PHP error messages show up on the screen when they occur?