I've had some hard time getting the personalized error pages on a fresh ISPconfig3 fresh install on Ubuntu Server9.10
Not sure if this was addressed before, i did a search in the forum and didn't find anything related..
thing is that when you hit a non-existent page, you get an ugly error message
The requested URL /er was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.12 (Ubuntu) Server at example.myserver.com Port 80
And on the vhost error log you'll find:
[Sat Mar 13 16:07:12 2010] [error] [client 126.96.36.199] File does not exist: /var/www/myserver.com/web/aaaaaa
[Sat Mar 13 16:07:12 2010] [error] [client 188.8.131.52] File does not exist: /usr/share/apache2/error/404.html
well here's the issue.. APACHE2 default configuration defines ErrorDocument in /etc/apache2/conf.d/localized-error-pages
so you might want to get rid of that's file content
Keep up the good works!