php undefined error and debugging on centos

I'm sorry, I'm relatively new to Centos and CWP, my problems mostly come from my poor knowledge of the various server settings.

The problem arises from an application that I developed using Chronoforms (and Chronoconnectivity) on Joomla.
When the applications becomes too big (difficult to say what is big here) it starts giving errors when I try to edit it. Usually the error is a 500 WSOD or a 504 error.
For a while I remedied by changing max_execution_time but I can't do that indefinitely.
The problem is probably in some not well tested or optimized code in the libraries used by Chronoforms and Chronoconnectivity.
So I wanted to try to understand in what file the error happens.
The problem happens on a vps with CWP6 and php 5.6 and on a vps with CWP7 Pro and php 7.2 .
I tried to edit the same app on another server (shared hosting of another ISP) and there's no  problem.
I checked the php.ini files of this last server and there's no apparent difference from the php.ini of the 2 vps.

Now I'm a little bit lost because I'm trying to get some info from log files but I don't understand where to go and check.
In my vps with cwp7 pro I'm using Nginx & apache and PHP-FPM selector (NEW) with php 7.2.
I changed the apache loglevel to trace6 but there's no info about my problem.
I changed max_execution_time on the php.ini of php-fpm 7.2 and I can see the right value in phpinfo(). But I can't get the error_log directive to work.
Whatever file name or path I put there I can't find the corresponding file, even searching the entire file system. I changed directory permissions, created empty files to no avail.

Now I'm lost, I don't know what to check and where to start, many information on the web refer to other setups and it's difficult to understand what to check or what parameters set.

Is there any direction that you can give me?

Thank  you
  maxx