Can't generate DKIM and access Server config

Hello, after update into 3.1.12, and later 3.1.13 i can't Generate DKIM Private-key for email domains, and i can;t access System -> System -> Server config -> My server domain.
My OS is CentOS release 6.9 (Final) 86_64, using default PHP Version 5.3.3 (Server API CGI/FastCGI) on Apache/2.2.15 (CentOS).
Here is what i found:
error.log
Server config
[Mon Sep 17 13:06:43 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/admin/server_config_edit.php on line 78, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:06:44 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/admin/server_config_edit.php on line 78, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:06:45 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/admin/server_config_edit.php on line 78, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:06:45 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/admin/server_config_edit.php on line 78, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:06:46 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/admin/server_config_edit.php on line 78, referer: https://serv.domain.com:8080/index.php

Generate DKIM
[Mon Sep 17 13:20:56 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/mail/ajax_get_json.php on line 43, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:20:57 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/mail/ajax_get_json.php on line 43, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:20:57 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/mail/ajax_get_json.php on line 43, referer: https://serv.domain.com:8080/index.php
[Mon Sep 17 13:20:57 2018] [warn] [client 59.xx.xx.xx3] mod_fcgid: stderr: PHP Parse error: syntax error, unexpected '[' in /usr/local/ispconfig/interface/web/mail/ajax_get_json.php on line 43, referer: https://serv.domain.com:8080/index.php

access.log:
Server config
59.xx.xx.xx3 - - [17/Sep/2018:13:06:43 +0200] "GET /admin/server_config_edit.php?id=1 HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:06:44 +0200] "GET /admin/server_config_edit.php?id=1 HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:06:45 +0200] "GET /admin/server_config_edit.php?id=1 HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:06:45 +0200] "GET /admin/server_config_edit.php?id=1 HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:06:46 +0200] "GET /admin/server_config_edit.php?id=1 HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"

Generate DKIM
59.xx.xx.xx3 - - [17/Sep/2018:13:20:56 +0200] "GET /mail/ajax_get_json.php?1537183255922&domain_id=emaildomain.com&dkim_public=&dkim_selector=default&type=create_dkim HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:20:57 +0200] "GET /mail/ajax_get_json.php?1537183256817&domain_id=emaildomain.com&dkim_public=&dkim_selector=default&type=create_dkim HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:20:57 +0200] "GET /mail/ajax_get_json.php?1537183257009&domain_id=emaildomain.com&dkim_public=&dkim_selector=default&type=create_dkim HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"
59.xx.xx.xx3 - - [17/Sep/2018:13:20:57 +0200] "GET /mail/ajax_get_json.php?1537183257209&domain_id=emaildomain.com&dkim_public=&dkim_selector=default&type=create_dkim HTTP/1.1" 500 - "https://serv.domain.com:8080/index.php" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:63.0) Gecko/20100101 Firefox/63.0"

Temporary i fixed Server config access by using previous version of server_config_edit.php, but i didn't have the ajax file, to do the same.
Any help is welcome, thank You.

 

Thank You for fast reply. I was thinking, that old PHP version may be a reason for that. I'm using some new PHP versions with PHP-FPM. Is it possible to make control panel use them instead of the default?

I've just updated base PHP into 5.6, and it's working fine now. Thank You once again for help.