[till]

There is a bug in the vhost template. In The file /usr/local/ispconfig/server/conf/vhost.conf.master around line 258 you will find these lines:

    suPHP_Engine on
    # suPHP_UserGroup <tmpl_var name='system_user'> <tmpl_var name='system_group'>
    AddHandler x-httpd-php .php .php3 .php4 .php5
    suPHP_AddHandler x-httpd-php

replace it with:

        suPHP_Engine on
        # suPHP_UserGroup <tmpl_var name='system_user'> <tmpl_var name='system_group'>
        AddHandler x-httpd-suphp .php .php3 .php4 .php5
        suPHP_AddHandler x-httpd-suphp

Then change a setting in the website in ispconfig and click on save so that the config gets rewritten.