Never edit a domain config file manually and using webmin to edit apache files can conflict with ispconfig. If you want to add code to a vhost config file, add it in the apache or nginx directives field of the webite in ispconfig.
The tutorial you followed is about a completely different topic and not suitable to display roundcube. Beside that, its from 2007and will mostnlikely not work with ispconfig 3.0.4 ans later anyway. I highly recommend to undo all changes you made according to this guide or you eill likely break your setup.