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
Thanks I don't know how it happened (the only thing i can think of was a power outage), but my /etc/network/interfaces suddenly looked like this and on top of it /etc/hosts after changing both back to perfect server guide specs everything's back to normal