Hairy, still cannot reproduce the error you are seeing on any of my kit (CentOS 6.4 x64)
For me, the APS installer was an added bonus into an already cracking interface and since it was added I have got around 40 different sites both internal and web-facing with packages downloaded and deployed using it.
I reckon a way forward for you would be to download some of the virtual images that you can access with subscription.
Check to see if that runs and gives same error - if it does then perhaps something environmental outside of your ispconfig server - eg. are you behind any restrictive firewall or proxy server.
If it doesnt give the error then points to build issue on your server.
I'd be checking/comparing anaconda.ks.cfg and install.log files to see what was installed at build time. Also /var/log/yum.log files to see what packages added on top, etc. A downloaded working version will at least give you a clue as to whats going on with yours.
I've been through a few control panels and regard ISPConfig as the best one for my purposes. Therefore happy to help others if I at all can.