500 - Internal Server Error - Related to Relationships

Hi to all,
When I am trying to access the Relationships node to Studio tree (in all modules) I get in Chrome console 500 - Internal Error, and nothing happens (I have to refresh the page in order to continue to work. The same happen also when I am trying to Quick repair and to build relationships. I have check multiple time and also real-time the suitecrm.log, php error log and apache2 log nothing is there.

I had also try to update to the latest 7.10.x version and 7.12.x but at the last step also produce the same error and the step stop and upgrade is incomplete.

My setup is as follows:
SVR - Ubuntu 14.04
CRM - 7.10.4
Apache 2
PHP - 5.6
MySQL - 5.6

I have also restored my setup to different server with Ubuntu 20.04, PHP - 5.6 and 7.3, MySQL - 8 with the same problem.

My setup have an extensive set of custom modifications using the studio tool (not code) so I cannot proceed to new installation.

While I have this serious problem, I cannot find a way to troubleshoot this, even with the β€˜debug’ option at suitecrm log there is nothing at server that shows an error in any log.

I would appreciate if someone can point me a direction how to troubleshoot and fix this problem (seems related to relationships).

Thank you in advance,
Nick