To remove this error from /var/log/apache2/error.log , you have to change ispconfig config.

In file /home/admispconfig/ispconfig/lib/config.inc.php change the line from:

$go_info["server"]["apache2_php"] = 'both';

to:

$go_info["server"]["apache2_php"] = 'addtype';

Change a website config (enable CGI scripts for example, save and disable it after) to apply change in Apache config.