I am no expert, but...
that happened to me last night as I was reinstalling a new server (from the perfect server debian +ISPConfig3). What was happening was that my existing DHCP server was set to hand out old local DNS server IPs. I editted the dhcp server to send only the external DNS servers and I was able to install the system with no issue.
I don't know if this is a similar situation for you, but good luck