I discuss using this method in the cited thread, too.
This indicates that Roundcube is configured to work only with your "default" site.
Did you read the link to the "common pitfall" thread that I linked to in the aforementioned post? That thread is about SquirrelMail, but almost everything therein applies to Roundcube, too.
Once you've read through that, if you still have issues, let us know.