Error 500: help with troubleshooting

["absolutely_free@xxxxxxxxx" <absolutely_free@xxxxxxxxx>]

Hi,

I am using Apache on a CentOS VPS:

# httpd -V

Server version: Apache/2.2.15 (Unix)

Server built:   Feb  9 2016 17:28:49

Server's Module Magic Number: 20051115:25

Server loaded:  APR 1.3.9, APR-Util 1.3.9

Compiled using: APR 1.3.9, APR-Util 1.3.9

Architecture:   64-bit

Server MPM:     Prefork

  threaded:     no

    forked:     yes (variable process count)

PHP is executed through php-fpm.

I often get Internal server error on my Apache logs:

xx.xx.xx.xx - - [13/Feb/2016:12:23:59 +0100] "GET /sometext/?refresh_cens HTTP/1.1" 500 79618

and few seconds later:

[Sat Feb 13 12:24:00 2016] [error] [client xx.xx.xx.xx] FastCGI: server "/usr/lib/cgi-bin/php5-fcgi" stderr: PHP message: PHP Fatal error:  Exception thrown without a stack frame in Unknown on line 0, referer: http://www.domain.it/sometext/?refresh_cens

So I think I have to solve Apache issue first.

LogLevel is "debug", but I have nothing in general error_log.

I need some help to start troubleshooting this issue, thank you