Below are the Error log - ( tail -f /usr/local/apache/logs/error_log | grep IP address )
[15-Nov-2016 10:43:07 UTC] PHP Fatal error: Uncaught Error: Call to undefined function mysql_connect() in /home/evokeete/public_html/wp-includes/wp-db.php:1561
#0 /home/user/public_html/wp-includes/wp-db.php(658): wpdb->db_connect()
W3TC\DbCache_WpdbInjection_Qu in /home/user/public_html/wp-includes/wp-db.php on line 1561
2. Check the '.htaccess file', if you'll find any code which is other than WordPress's default '.htaccess' file code then please comment that code.
3. Check files and directories permission and ownership.
4. Check that your website is not triggering any mod security rules.
5. Please check the the issue with theme and plugin provider.
6. Check your website is not exceeding resource limit/inode limit allocated to it.
7. If all above is fine then please remount the "cagefs" - cagefsctl --remount-all
It fixed my issue!