I then went to the online demo of ISPConfig at http://demo3.ispconfig.org/ for fun just to look and see how the APS Installer works in action. The demo version gives the similar error that I am having.
The demo is not a ispconfig installation, so downloading any packages can not work of course as we disabled that. The demo is a modified version of the interface pages to give a impression how ispconfig lookslike when it is installed. It does not have a server part nor does it contain all scripts
I installed the latest ispconfig version on centos 6.3 and the aps installer works flawlessly. I used the vmware download of the perfect setup for this test, so it is a exact copy of the perfect setup guide without any modifications except of updating ispconfig to 126.96.36.199 first.