Originally Posted by Hans
Is the IP-address 192.168.1.75 also mentioned within your file /etc/network/interfaces and within ISPConfig under Management > Server > Settings?
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
iface lo inet loopback
# The primary network interface
# allow-hotplug eth1
iface eth1 inet static
eth0 is the onboard 10/100 which I don't use, I do use eth1 which is a 1000mbit card.
It is also mentioned under management > server under IP address