In the beginning of the HowTo I mention :
If you want to access ISPConfig Panel and webmail through SSL I also assume that you followed the chapter 6.2 Enabling SSL For The ISPConfig Web Interface of The ISPConfig 3 manual (this is only needed if you want to access the control panel AND the webmail interface through ssl on port 8080).
If you followed the above, then I finally assume that you read the workaround in this post.
You HAVE to do those things to have SSL over port 8080. After that you can follow anyone of the 2 links in my previous post.