check your dns and any firewalling?
Cannot offer an exact fix, just what I came across...
I just upgraded a few versions on CentOS hardware that did not have the APS functionality in and on one machine got a similar error to you.
I did 3 fresh installs and it all works first time out of the box. Other glitches I came across have nothing to do with APS installer.
On my Centos 6.4 x86_64 architecture it turns out that the machine that was failing did not have proper dns resolution. This was traced to an update of the bind package that inserts a line into /etc/named.conf looking for a non-existent file (/etc/named.conf.local). So named refused to start. On clicking "update package list" the machine had no dns and therefore couldnt grab the list.
Once resolved ("touch /etc/named.conf.local" and "service named restart") it worked. This is a distro problem and NOT an ispconfig problem.
Not saying thats your issue, just describing what happened my end.