You can set the overrides either in your apache2.conf file, in the vhost master template or in the apache directives field.
How would I allow a client to override the ErrorDocuments in the .htaccess file? what do I change in the vhost.master?
You dont have to edit anything in .htaccess or vhost master to override the error documents. Just enable the checkbox in the site settings and the client can cahge the error pages in the error directory of ist website.
drupal wants to be able to handle its own error documents. How would I enable this ability? Drupal says to do it in the .htaccess file, but apache says it isn't allowed.
Then try to add: <Directory /var/www/yoursite.com/web> AllowOverride FileInfo </Directory> in the apache directives field of the website, you will have to replace the path with the path of your site.
Or if you want enable .htaccess for your client in some website put this in directives apache <Directory /var/www/domain.ltd/web> Options +Includes -Indexes -ExecCGI AllowOverride All Order allow,deny Allow from all </Directory>
All harmless settings are already allowed for .htaccess, only if you want to give a client the ability to change settings in .htaccess that might be used to extend the permissions and performance of his webspace you will have to declare that manually. You should always be aware that this might enable the website owner to get more and higher resources from your server then the settings that you have set in ispconfig for the website.
You should never use "AllowOverride All" on a production system!!! This enables you to change nearly everything in the config! For example renenable disabled functions, to break the open basedir jail and so on... It is a really bad idea!!! Regards, Andreas